Semiconductor device having a high current gain and a higher Ge amount at the base region than at the emitter and collector regions, and photoelectric conversion apparatus using the device

ABSTRACT

A semiconductor device comprises at least an emitter region of a first conductivity type, a base region of a second conductivity type, and a collector region of a first conductivity type. The base region essentially consists of Si 1-X  Ge X  (0&lt;X&lt;1), further comprises regions formed in a depletion layer close to an interface between the base region and the collector region or in the collector region and in a depletion layer close to an interface between the base region and the emitter region or in the emitter region, and has a larger Ge amount at the base region-side and a smaller Ge amount at the emitter and collector sides.

This application is a continuation of application Ser. No. 08/215,665 filed Mar. 22, 1994, now abandoned which is a continuation of application Ser. No. 07/927,056 filed Aug. 10, 1992, now abandoned, which is a divisional of application Ser. No. 07/448,211 filed Dec. 8, 1989, issued as U.S. Pat. No. 5,159,424 on Oct. 27, 1992.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor device and a photoelectric conversion apparatus using the device.

2. Related Background Art

Conventionally, wide variety of configurations and structures are used as the semiconductor device. Among them, a bipolar transistor (hereinafter, referred to as "BPT") has many advantages that high speed switching operation can be expected.

A DOPOSBPT (Doped Poly Silicon BPT), i.e., a BPT having a polysilicon emitter, is known as a BPT having a shallow junction and a high packing density.

FIG. 1 is a sectional view showing an example of a conventional BPT. Referring to FIG. 1, reference numeral 1 denotes a substrate made of silicon (Si); 2, an n⁺ -type buried region including material impurity making conductivity into n-type within Si; 3, an n⁻ -type region having a low impurity concentration; 4, a p-type region including material (impurity) making conductivity into p-type serving as a base region; 5, an n⁺ -type region including n-type impurity 1 serving as an emitter region; 6, an n-type region serving as a channel stopper; 7, an n⁺ -type region of high impurity concentration for reducing a collector resistance of a bipolar transistor; 101, 102, 103, and 104, insulating films for insulating the element, electrodes, and wirings; 200, an electrode made of a metal, silicide, polycide, or the like.

The substrate 1 has an n conductivity type upon doping of an impurity such as phosphorus (Ph), antimony (Sb), or arsenic (As), or a p conductivity type upon doping of an impurity such as boron (B), aluminum (Al), or gallium (Ga). The buried region 2 need not be necessarily formed. Boron (B), gallium (Ga), or aluminum (Al), and germanium (Ge) are doped in the base region 4. The emitter region 5 consists of polysilicon formed by low-pressure chemical vapor deposition (LPCVD).

The base region 4 consists of a mixed crystal of silicon (Si) and germanium (Ge). Both of Si and Ge are diamond type crystals and perfect solid solutions. Therefore, Si_(1-X) Ge_(X) is a perfect diamond type crystal for every X (0 to 1). A forbidden band width E is about 1.1 eV for Si and about 0.7 eV for Ge.

FIG. 2 is a graph showing a relationship between a mixed crystal ratio X and the forbidden band width Eg of Si_(1-X) Ge_(X). Referring to FIG. 2, the abscissa represents the mixed crystal ratio X, the ordinate represents the forbidden band width Eg, a reduction width ΔE_(C) at a conduction band side, and a reduction width ΔE_(V) at a valence band side. As is apparent from FIG. 13, in Si_(1-X) Ge_(X), most of a band gap reduction occurs in a valence band. This is very convenient for the hetero BPT because injection of electrons from an emitter to a base is not prevented.

In another configuration, the BPT of this type comprises a polysilicon emitter region. A silicon oxide having a thickness of 10 to 20 Å is formed between the polysilicon emitter region and an emitter region formed by diffusing an impurity from polysilicon to single-crystal silicon.

FIG. 3 is a schematic sectional view showing a conventional BPT. Referring to FIG. 3, reference numeral 1 denotes a substrate; 2, an n⁺ -type buried region; 3, an n⁻ -type region having a low impurity concentration; 4, a p-type region serving as a base region; 5, an n⁺ -type region serving as an emitter region; 6, an n-type region serving as a channel stopper; 7, an n⁺ -type region for reducing a collector resistance of the bipolar transistor; 8, oxide regions formed in the single-crystal emitter region 5; 101, 102, 103, and 104, insulating films for isolating the element, electrodes, and wirings; and 200, electrodes made of a metal, a silicide, a polycide, or the like.

The substrate i has an n conductivity type upon doping of an impurity such as phosphorus (Ph), antimony (Sb), or arsenic (As), or a p conductivity type upon doping of an impurity such as boron (B), aluminum (Al), or gallium (Ga). The buried region 2 need not be necessarily formed. The n⁻ -type region 3 is formed by epitaxial techniques. Boron (B), gallium (Ga), or aluminum (Al), and germanium (Ge) are doped in the base region 4. The emitter region 5 consists of polysilicon formed by low-pressure chemical vapor deposition (LPCVD).

This BPT, i.e., the BPT having the single crystal emitter region and the silicon oxide formed between the polysilicon emitter region and the base region, has an advantage in that the oxide film can increase a current gain of the BPT.

Analytically theoretical expressions will be described with reference to FIG. 4 to explain the reason why the oxide film can increase the current gain of the BPT.

FIG. 4 shows a diagram showing potentials in the direction of depth of the cross section along the line A--A' in FIG. 3 in a normal operation. Referring to FIG. 4, reference numeral W_(E) denotes a thickness of an emitter neutral region; W_(B), a thickness of a base neutral-region. As shown in FIG. 4, since the oxide is formed between the emitter region and the base region, a potential barrier is present at a position of W.sub. E'.

In the conventional semiconductor device having the above structure, the base current consists of the following two components.

A diffusion current of positive holes flowing from the base to the emitter can be approximated as follows due to the presence of the potential barrier:

    J.sub.B1 =(q·n.sub.i.sup.2 ·D.sub.P /N.sub.E ·L.sub.P)×tan h(W.sub.E '/L.sub.P) exp(V.sub.BE /kT)-1!(1)

A recombination current of electrons injected from the emitter is represented as follows: ##EQU1## A collector current is represented as follows:

    J.sub.C =(q·n.sub.i.sup.2 ·D.sub.n N.sub.B ·L.sub.n) cosech(W.sub.B /L.sub.N)!× exp(V.sub.BE /kT)-1!(3)

where q is a charge, n_(i) is an intrinsic semiconductor charge density (Si), N_(E) is an emitter impurity density, N_(B) is a base impurity density, D_(P) is a positive hole diffusion coefficient, D_(N) is an electron diffusion coefficient, L_(P) is a positive hole diffusion length (ε(D_(P) τ_(P))^(1/2)), L_(N) is an electron diffusion length (≈(D_(N) τ_(N))^(1/2)), k is a Boltzmann constant, T is an absolute temperature, VBE represents base-emitter forward biased electrons, τ_(P) and τ_(N) are minority carrier lifetimes of the positive holes and the electrons.

In equation (1), when W_(E) ' is decreased, J_(B1) is reduced. Therefore, a current gain can be increased:

    h.sub.FE =J.sub.C /(J.sub.B1 +J.sub.B2)                    (4)

FIG. 5 is a schematic sectional view showing a different type of BPT. Referring to FIG. 5, reference numeral 1 denotes a substrate; 2, an n⁺ -type buried region; 3, an n⁻ -type region having a low impurity concentration; 4, a p-type region serving as a base region; 5, an n+-type region serving as an emitter region; 6, an n-type region serving as a channel stopper; 7, an n⁺ -type region for reducing a collector resistance of the bipolar transistor; 101, 102, 103, and 104, insulating films for insulating the element, electrodes, and wirings; and 200, electrodes made of a metal, a silicide, a polycide, or the like.

The substrate 1 has an n conductivity type upon doping of an impurity such as phosphorus (Ph), antimony (Sb), or arsenic (As), or a p conductivity type upon doping of an impurity such as boron (B), aluminum (Al), or gallium (Ga). The buried region 2 need not be necessarily formed. The n⁻ -type region 3 is formed by epitaxial techniques. Boron (B), gallium (Ga), or aluminum (Al), and germanium (Ge) are doped in the base region 4. The emitter region 5 consists of polysilicon.

In the conventional semiconductor device having the above structure, the base current consists of the following two components.

A diffusion current of positive holes flowing from the base to the emitter can be approximated as follows due to the presence of the potential barrier:

    J.sub.B1 ={(q·n.sub.i.sup.2 ·D.sub.p)/(N.sub.E ·L.sub.p)}×cosh(W.sub.E /L.sub.p){exp(V.sub.BE /kT)-1}(1)

A recombination current of electrons injected from the emitter is represented as follows: ##EQU2##

A collector current is represented as follows:

    J.sub.C ={(q·n.sub.i.sup.2 ·D.sub.n)/(N.sub.B ·L.sub.n)}×cosech(W.sub.B /L.sub.np){exp(V.sub.BE /kT)-1}(3)

where q is a charge, n_(i) is an intrinsic semiconductor charge density (Si), N_(E) is an emitter impurity density, N_(B) is a base impurity density, D_(P) is a positive hole diffusion coefficient, D_(N) is an electron diffusion coefficient, L_(P) is a positive hole diffusion length (˜(D_(P) τ_(P))^(1/2)), L_(N) is an electron diffusion length (D_(N) τ_(N))^(1/2)), k is a Boltzmann constant, T is an absolute temperature, V_(BE) represents base-emitter forward biased electrons, W_(B) is a thickness of the base, and W_(E) is a thickness of the emitter. Note that τ_(P) and τ_(N) are minority carrier lifetimes of the positive holes and the electrons.

In the above conventional BPT, however, a point defect as an electrical recombination center or dislocation caused by a lattice defect occurs in an interface between the Si crystal forming the emitter region and the Si_(1-X) Ge_(X) eutectic forming the base region. Therefore, a defect occurs near an emitter-base junction or between the base and the collector to increase a base current of the BPT, thereby reducing a current gain h_(FE).

An influence of the point defect or dislocation on h_(FE) is significantly increased at a low current side of a collector current, and h_(FE) is reduced close to one or below one in some cases.

This will be described in detail below.

The point defect or dislocation at the interface between the Si crystal or Si_(1-X) Ge_(X) mixed crystal is caused by a difference in lattice coefficients between Si and Si_(1-X) Ge_(X).

The lattice coefficient of Si is d_(Si) =5.43086 Å, while that of Ge is d_(Ge) =5.65748 Å, i.e., a lattice coefficient difference is about 4%. Therefore, lattice coefficient values of the Si crystal and the Si_(1-X) Ge_(X) mixed crystal are different. For this reason, a stress occurs in the interface between the two crystals to partially cut a chemical combination between elements at the interface. This is a socalled point defect. When the point defect significantly occurs, dislocation is caused.

A constant relationship is present between the mixed crystal ratio X of Ge and the-thickness of a layer in which no dislocation occurs. FIG. 6 is a graph showing this relationship. Note that this data is obtained when Si_(1-X) Ge_(X) is deposited on an Si substrate by the MBE method. Since growth is performed at 510° C., a transition region from Si_(1-X) Ge_(X) to Si is very thin.

In a layer having a homogeneous mixed crystal composition X, dislocation occurs in the interface if the thickness is not less than that of a hatched region shown in FIG. 6.

Next, an impurity concentration of the emitter region 5 in the conventional BPT and the conventional DOPOSBPT falls within the range of 10¹⁹ to 10²¹ cm⁻³ ; an impurity concentration of the base region, 10¹⁶ to 10¹⁸ cm⁻³ ; an impurity concentration of the collector region; about 10¹⁴ to 10¹⁶ cm⁻³.

In such a BPT, since the impurity concentration of the emitter region is higher (10¹⁹ cm⁻³ or more), narrowing of the band gap occurs, and injection efficiency of carriers from the emitter to the base is degraded (i.e., a current gain h_(FE) is decreased).

When an impurity concentration of a semiconductor is increased, a free carrier density is increased (i.e., minority carrier mobility is gradually decreased, and the band gap (forbidden gap width) is decreased). When the impurity density is 10¹⁷ to 10¹⁸ or more, band tailing occurs from the ends of the conduction and valence bands in an n- or p-type semiconductor.

FIGS. 7 and 8 are views showing band structures is semiconductors. Energy is plotted along the ordinate and a state density n(E) (i.e., the number of carriers per unit volume) is plotted along the abscissa in each of FIGS. 7 and 8. FIG. 7 is a view showing a band structure of an n-type semiconductor having a low impurity density. In this case, an n-type donor level is separated from a conduction band. However, FIG. 8 shows a semiconductor containing a high impurity density, and the width of a donor level is increased. The donor level becomes a donor band whose energy is coupled to that of the conduction band unique to the semiconductor. That is, as shown in FIG. 8, a degenerate conduction band is formed. As a result, band-end tailing occurs, and the band gap is changed from E_(g) to E_(g) ', thus causing band narrowing of ΔE_(g) =E_(g) -E_(g) '.

Band narrowing of the n-type semiconductor has been described in FIG. 8. Similar band tailing occurs on the side of the valence band in a p-type semiconductor having a high impurity concentration, and narrowing of a band width (prohibition band) E occurs.

A band-narrowing value is approximated as follows:

    ΔE.sub.g ={3q.sup.2 /(16πε.sub.s)}{(q.sup.2 ·N)/(ε.sub.s kT)}.sup.1/2                ( 4)

where q is a charge, ε_(s) is a semiconductor dielectric constant, k is a Boltzmann constant, T is an absolute temperature, and N is an impurity density.

A band-narrowing value of Si at room temperature is given as follows:

    ΔE.sub.g =22.5(N/10.sup.18).sup.1/2 meV              (5)

For example, if N=10¹⁸ cm⁻³, then ΔE_(q) =22.5 meV.

FIG. 9 is a graph showing results of band-gap narrowing width calculations by using equation (5) when boron (B) is doped in Si in a high concentration. The impurity density (cm⁻³) of the impurity is plotted along the abscissa in FIG. 9, and the narrowed width ΔE_(g) (meV) of the band gap is plotted along the ordinate in FIG. 9.

An essential carrier density n_(i) '² upon occurrence of band-gap narrowing is given as follows as compared with the carrier density without narrowing:

    n.sub.i '.sup.2 =n.sub.i.sup.2 exp(ΔE.sub.g /kT)     (6)

Since W_(B) <L_(n) and J_(B1) >J_(B2) are established, h_(FE) is approximated as ##EQU3## The current gain can be expressed when only band-gap narrowing is taken into consideration:

    h.sub.FE ≈h.sub.FE0 ·exp{(ΔE.sub.gb -ΔE.sub.ge)/kT}                                     (8)

where h_(FE0) is a current gain without band-gap narrowing.

The conventional BPT had an emitter impurity concentration of about 10²⁰ to 10²¹ cm⁻³ and a base ts reason, AEg_(e) is large, and AEg_(b) is almost zero. impurity concentration of about 10¹⁶ to 10¹⁸ cm⁻³. For this reason, ΔE_(ge) is large, and ΔE_(gb) is almost zero. Therefore, the current gain h_(FE) is smaller than the designed value.

When the current gain h_(FE) is to be increased in a shallowed BPT, the impurity concentration of the ase must be increased. When the impurity concentration of the base is, however, increased, the base-emitter breakdown voltage is decreased. In addition, the base-emitter capacitance is undesirably increased.

In a photoelectric transducer apparatus using a conventional BPT, a frequency f_(T) is only about 1 GHz. The photoelectric transducer apparatus of this type is incompatible with HD (high Division; an area sensor coping with high vision).

To the contrary, as a conventional method of increasing the current gain h_(FE) , a silicon oxide film having a thickness of 10-20 Å has been formed between emitter region and base region. Such BPT can prevent hall from introducing from base region into emitter region due to a potential barrier formed within a valence band by an oxide film at an interface between the base and emitter. Therefore, the advantage that the current gain can be increased is obtained.

FIG. 10 shows a potential during a normal operation at A-A' sectional area wherein a silicon oxide is formed between the emitter and base regions of semiconductor device in FIG. 6. In the drawing, W_(E) denotes a thickness of the emitter neutral area, W_(B) denotes a thickness of the base neutral area. Further, as shown in the drawing, since a silicon oxide is formed between the emitter and base regions, a potential barrier is formed at a position of W_(E).

However, since the potential barrier due to the oxide film is formed not only at the conduction band but also at the valence band, it would be an obstacle against an electron stream as a carrier in the emitter, thereby emitter resistance is formed. Therefore, also it would be an obstacle against a collector current I_(c) stream. It would be a cause of inclining the current property of current gain h_(FE), i.e. h_(FE) depends upon base-emitter voltage V_(BE).

It is also difficult to obtain equal contents of oxygen in oxide films. The oxygen contents vary depending on individual BPTs, and therefore variations in BPT characteristics occur, resulting in inconvenience.

In particular, the characteristic variations of the individual BPTs are very important when they occur in a linear IC, a line sensor, and the like.

SUMMARY OF THE INVENTION

The present invention resides in providing a solution to the above described problems. That is, the present invention provides a semiconductor device not producing a point defect or dislocation in the hetero interface.

The present invention provides a semiconductor device which has a high current gain h_(FE), can prevent a decrease in base-emitter breakdown voltage, and has a small capacitance, and a photoelectric transducer apparatus using this semiconductor device.

It is an object of the present invention to provide a semiconductor device having a high current gain and less characteristic variations, and a photoelectric transducer apparatus using the same.

Further, the present invention provides a semiconductor device which can minimize characteristic variations and has a high current gain h_(FE) and a high frequency, and a photoelectric transducer apparatus using the same.

The object of the present invention is to provide a semiconductor device characterized by comprising at least an emitter region of a first conductivity type, a base region of a second conductivity type, and a collector region of a first conductivity type, the base region essentially consisting of Si_(1-X) Ge_(X) (0<X<1), further comprising regions formed in a depletion layer close to an interface between the base region and the collector region or in the collector region and in a depletion layer close to an interface between the base region and the emitter region or in the emitter region, and having a larger Ge amount at the base region side and a smaller Ge amount at the emitter and collector sides.

Another object of the present invention is to achieve in above semiconductor device, that Si_(1-X) Ge_(X) is 0.0625≦X ≦0.375.

The further object of the present invention is to provide a semiconductor device, characterized in that addition of Ge is performed by ion implantation.

The still further object of the present invention is to provide a photoelectric transducer apparatus characterized by using a semiconductor device.

The next object of the present invention is to provide a semiconductor device at least having an emitter region of a first conductivity type, a base region of a second conductivity type, and a collector region of a first conductivity type, characterized in that

a at least B and Ge are doped in the base region; and

b N_(D)≦ 10¹⁹ cm⁻³ and N_(Ge) >N_(B) where N_(D) is an impurity concentration near a junction surface of the emitter region contacting the base region, N_(B) is a B concentration of the base region, and N_(Ge) is a Ge concentration in the base region.

The further object of the present invention is to provide a semiconductor device, characterized in that B and Ge are doped in the base region by ion implantation.

The still further object of the present invention is to provide a semiconductor device, characterized in that N_(D) falls within a range of 10¹⁷ to 10¹⁹ cm⁻³.

The still further object of the present invention is to provide a semiconductor device, characterized in that an Si layer having an impurity concentration of 10¹⁹ to 10²¹ cm⁻³ is formed between the emitter region and an electrode.

The still further object of the present invention is to provide a semiconductor device, characterized in that condition N_(D) <N_(B) is established.

The still further object of the present invention is to provide a photoelectric transducer apparatus using the semiconductor device of the first to fifth gists.

An object of the present invention is to provide a semiconductor device characterized by a bipolar transistor structure in which an emitter region is deposited on a base region,

wherein the emitter region is formed after an oxide on the base region is removed, and the emitter region essentially consists of a single crystal.

An object of the present invention is to provide a semiconductor device of the present invention, which has at least an emitter of a first conduc.tivity type, a base of a second conductivity type, and a collector of the first conductivity type,

is characterized in that at least said base essentially consists of an Si_(1-X) Ge_(X) crystal (0<X<1),

an interface between the Si_(1-X) Ge_(X) crystal and an Si crystal is an inclined hetero interface, and

a depth of Si_(1-X) Ge_(X) is the same as or larger than a depth of said base.

In the above characteristic, in at least said base region, a Ge composition X is smaller in a shallow portion and larger in a deeper portion.

In the above characteristic, doping of Ge is performed by ion implantation.

A photoelectric transducer apparatus of the present invention is characterized by using the above semiconductor device.

Further object of the present invention is to provide a semiconductor device characterized by comprising at least an emitter region of a first conductivity type, a base region of a second conductivity type, a collector region of the first conductivity type, and an emitter electrode,

wherein the emitter region essentially consists of a single crystal,

a carrier concentration at least near an interface between emitter region and the base region in the emitter region is not more than 1×10¹⁹ cm⁻³,

an emitter region having a high impurity concentration of 1×10¹⁹ to 1×10²¹ cm⁻³ is formed between the emitter electrode and the emitter region, and

B and Ge are doped as impurities in the base region, and a Ge concentration is higher than a B concentration.

In the above characteristic part, the impurity concentration of the base region is not less than 1×10¹⁸ cm⁻³.

In the above characteristic part, the impurity concentration of the base region is not less than 1×10²⁰ cm⁻³.

In the above characteristic part, the Ge concentration is 8.25 times the B concentration.

In the above characteristic part, when an impurity concentration near an interface with the emitter region having the high impurity concentration in the emitter region is defined as N_(E1) and an impurity concentration of the emitter region having the high impurity concentration is defined as N_(E2), condition N_(E2) >eN_(E1) is satisfied.

A photoelectric transducer apparatus is characterized by using the semiconductor device having the above characteristic part.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1, 3 and 5 show schematic sectional views of examples of conventional bipolar transistors (BPT);

FIG. 2 shows a graph explaining relation between mixed crystal ratio X and band gap width Eg of Si_(1-X) Ge_(X) ;

FIG. 4 shows a potential of normal operation of A-A' section of BPT in FIG. 3;

FIG. 6 shows a graph explaining a relation between a mixed crystal ratio X and a thickness of a layer without dislocation;

FIGS. 7 and 8 show a band structure of the semiconductor;

FIG. 9 shows a graph of a measured value of band gap narrowing width in case of highly doping of B into Si;

FIG. 10 shows a potential during a normal operation at a sectional area A-A' in FIG. 5 in case of forming the silicon oxide between the emitter and base regions of BPT in FIG. 5;

FIG. 11 shows a potential barrier with a problem of prior art hetero BPT;

FIG. 12 shows a relation between ΔEg and maximum value (f_(T) max) of f_(T) ;

FIG. 13 shows a diffusion coefficient of B, Ga, Al, or the like to be a p type impurity;

FIG. 14 shows a graph of diffusion coefficient of Ge;

FIGS. 15 and 16.show an upper limit and desirable lower limit of N_(D) ;

FIGS. 17A and 17B show a band structure of the semiconductor;

FIG. 18 shows a graph of band gap narrowering in case of highly doping of B into Si;

FIG. 19 shows a graph of solubility of impurity into solid state Si;

FIG. 20 shows a graph of effect of Ge to the band gap;

FIG. 21 shows a potential during a normal operation at A-A' section of BPT in FIG. 48;

FIG. 22 shows a graph of a relation between h_(FE) and V_(BE) ;

FIG. 23 shows a graph of relation between the mixed crystal ratio X of Ge and a thickness of layer without phase transition;

FIG. 24 shows a graph of an example of impurity Gp distribution at A-A' section in FIG. 25;

FIG. 25 shows a schematic sectional view of BPT structure;

FIG. 26 shows a relation collector current limit J_(c) and delay time I_(ec) of carrier from emitter into collector;

FIGS. 27A and 27B show, in this order, a graph of relation between the impurity concentration and mobility (μn, μp), and a graph of relations between base impurity concentration and lower limit of emitter impurity concentration;

FIG. 28 shows a relation between base impurity concentration N_(D) and depletion layer width WBC between base and collector and base depletion layer width X_(p) ;

FIG. 29 shows a graph of relation between a normalized position X/W_(B) within base and relative carrier number n^(x) ;

FIG. 30 is a graph which shows schematically V_(BF) dependency of h_(FE) ;

FIG. 31 shows a relation between fermi level E_(t) and carrier concentration within

FIG. 32 shows potential distribution of high concentration region;

FIG. 33 shows a graph of relation lower limit of emitter concentration and base concentration at W_(B) =1;

FIG. 34 shows a relation between depletion layer thickness and the width thereof;

FIG. 35, 38, 39, 40, 42, 44, 45, 46, 48, 50, 51, 53, 54, 55, and 56 respectively show schematic sectional view of BPT according to embodiments of the present invention;

FIG. 36 shows a graph of an example of impurity Ge distribution at A-A' section in FIG. 35;

FIG. 37 shows property of BPT improved by the present invention;

FIG. 41 shows a schematic circuit diagram for explaining a photoelectric conversion apparatus using BPT according to the present invention;

FIG. 43 shows B, Ge and P distributions in FIG. 42;

FIG. 47 shows a graph of lower and upper limits of N_(D) ;

FIG. 49 shows a graph of relation between h_(FE) and h_(BE) of BPT in FIG. 48;

FIG. 52 shows potential of BPT of Embodiment 16; and

FIG. 57 shows a schematic circuit diagram of photoelectric conversion apparatus according to Embodiment 16.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will be described by the description of the preferred embodiments made with reference to drawings as follows.

According to the present invention, a conventional problem of a point defect or dislocation in a hetero interface between Si forming an emitter region and Si_(1-X) Ge_(X) forming a base region can be solved, thereby increasing a current gain h_(FE) and a frequency f_(T).

In the present invention, an emitter is made of Si, and a base is made of Si_(1-X) Ge_(X). Therefore, Si_(1-X) Ge_(X) is E_(g) '=E_(g) -ΔE_(g) on the basis of a band gap E_(g) of Si, i.e., the band gap is reduced by ΔE_(g). Analytically theoretical expressions of a base current and a collector current in a BPT of the present invention are as follows (note that the end of the emitter brought into contact with a metal).

The base current mainly consists of two components, i.e., a diffusion current of positive holes from the base to the emitter:

    J.sub.Binj =(qn.sub.i.sup.2 D.sub.P /N.sub.E L.sub.P)×coth(W.sub.E /L.sub.p) exp(V.sub.BE /kT)-1!                            (2)

and a recombination current of electrons injected from the emitter: ##EQU4## The collector current is given as follows: ##EQU5## where q is a charge, n_(i) is an intrinsic semiconductor charge density (Si), N_(E) is an emitter impurity density, N_(B) is a base impurity density, D_(P) is a positive hole diffusion coefficient, D_(N) is an electron diffusion coefficient, Lp is a positive hole diffusion length (≅(D_(p) τ_(p))^(1/2)), L_(N) is an electron diffusion length (≅(D_(N) τ_(N))^(1/2)), k is a Boltzmann constant, T is an absolute temperature, V_(BE) is a base-emitter forward biased electron, and τ_(p) and τ_(N) are minority carrier lifetimes of the positive holes and the electrons.

When ΔE/kT>>1, J_(Bing) <<J_(Brec) is obtained. When W_(B) <<L_(N), h_(FE) is determined by the following relation:

    h.sub.FE ≈2(L.sub.N /W.sub.B).sup.2                (5)

That is, ΔE_(g) no longer has a large influence on h_(FE). Therefore, h_(FE) is uniquely determined by the value of (L_(N) /W_(B)).

When the Si_(1-X) Ge_(X) layer is shallower than the p-type layer, a potential of the BPT is obtained as shown in FIG. 11. Referring to FIG. 11, a forward bias is applied between the emitter and the base, and a reverse bias is applied between the base and the collector. Since the Si_(1-X) Ge_(X) layer is formed halfway in the p-type region, a potential barrier of ΔE_(g) is formed in the base. When the potential barrier as shown in FIG. 11 is formed, an increase of exp(ΔE_(g) /kT) in the collector current represented by equation (4) disappears. Minority carriers propagating in the base are interrupted by the potential barrier at a point A in FIG. 11. Carriers passing through the barrier become exp(-ΔE_(g) /kT). Therefore, the collector current is the same as that of a BPT of a normal homojunction.

Since an amount of minority carriers injected in the base is exp(ΔE_(g) /kT) times that of Si, the recombination current becomes larger than that of a homojunction. For this. reson, h_(FE) of the BPT becomes smaller than that of the homojunction BPT. Therefore, the interface between Si_(1-X) Ge_(X) and Si must be formed in the depletion layer of the base and collector junction or in the collector. Alternatively,. the interface must be formed in the depletion layer of the emitter and the base or at the emitter side. In this manner, diffusion of the minority carriers injected from the base to the emitter can be interrupted by the barrier at the hetero interface. That is, the hetero interface between Si and Si_(1-X) Ge_(X) must not be formed at a neutral region of a region serving as a base.

Next, according to the present invention, in Si_(1-X) Ge_(X), a Ge concentration X is preferably represented by:

    0.0625≦X≦0.375

An amount of electrons injected in the base is represented by: exp(ΔE_(g) /kT)

If ΔE/kT=1, an amount of the electrons injected in the base is about 2.7 times that of a conventional device, and h_(FE) is about twice that of the conventional device.

Therefore, in order to obtain a sufficient nondestructive degree, ΔE_(g) ≧kT is preferred. Since ΔE_(g) >0.025 eV at room temperature, the Ge concentration X=0.025/0.4=0.0625. Therefore, X is preferably 0.0625 or more. Note that if the device is used at a low temperature, sufficient characteristics can be obtained even by a smaller X.

X is preferably 0.375 or less.

FIG. 12 is a graph showing a relationship between ΔE_(g) and a maximum value (f_(Tmax)). As is apparent from FIG. 9, since f_(T) is not improved when ΔE_(g) >0.15 eV, ΔE_(g) need only be 0.15 eV or less.

The value of X for obtaining ΔE_(g) ≦0.15 eV is X≦0. 375.

As described above, when the device is used at room temperature, the Ge concentration X is preferably: 0.0625≦X≦0.375.

Doping of Ge in the semiconductor device of the present invention is preferably performed by ion implantation.

Conventionally, since a device is formed by an MBE method or the like, a stepwise hetero is formed.

That is, conventionally, a boron region for defining a base and an Si--Ge region are simultaneously formed by the MBE method or the like. Thereafter, however, when annealing is performed in a formation process, the region of boron as an impurity and the Si--Ge region do not coincide with each other.

According to measurement performed by the present inventor, de diffusion in Si is very slow. Especially at 1,000° C. or less, Ge was not almost diffused and therefore could not be measured. At 1,000° C., D=3×10⁻¹⁴ cm³ /sec was obtained. As shown in FIG. 13, a diffusion coefficient of B, Ga, Al, or the like serving as a p-type impurity is much larger than that of Ge. FIG. 14 shows data obtained by the present inventor to examine the diffusion coefficient of Ge. When diffusion data is examined on the basis of the gradient of the graph, the Ge diffusion coefficient is assumed to be substantially the same as that of As indicated by A.

In the BPT, an emitter concentration is as very high as 10¹⁷ to 10²¹ cm⁻³. Therefore, when annealing is performed, a junction moves from the emitter to the base. Since, however, the p-type side of the base-collector junction becomes deeper than the case of Si--Ge, this base-collector junction becomes inconvenient for a BPT.

Therefore, Ge doping in the semiconductor device of the present invention is preferably formed by ion implantation.

According to the present invention, F_(T) of a photoelectric transducer apparatus can be improved. In addition, a nondestructive degree can be improved by increasing a current gain.

Therefore, there is provided a solid-state image pickup apparatus capable of reading out data from a single element a plurality of times without performing correction and capable of performing high-speed reading.

According to the present invention, there is provided a semiconductor device having a high base-emitter breakdown voltage, a small base-emitter capacitance, and a high h_(FE), and a photoelectric transducer apparatus using the same.

The function of the present invention will be described in detail with reference to constituting elements.

According to the present invention

1 The impurity concentration near the junction surface between the emitter region and the base region is decreased to 10¹⁹ cm⁻³ or less to prevent band-gap narrowing and hence prevent a decrease in current gain.

The impurity concentration of 10¹⁹ cm⁻³ is kept at at least a portion near the junction surface between the emitter and base regions.

The impurity concentration of the emitter region is kept at 10¹⁹ cm⁻³ due to the following reason.

When the impurity concentration of the base region is set high, the breakdown voltage of the junction between the base and the emitter region is decreased.

When a semiconductor device is used in a normal IC, a minimum breakdown voltage is 2 V or more. However, when a sufficient margin is to be taken into consideration, a minimum breakdown voltage is preferably 3 V. Therefore, a case wherein the minimum breakdown voltage is 3 V will be exemplified below.

A p-n junction breakdown voltage is determined by avalanche amplification when the impurity concentration of an emitter region is 10¹⁷ cm⁻³ or less. However, when the impurity concentration of the emitter region is 10¹⁷ cm⁻³ or more, an influence of a tunnel current is enhanced. When the impurity concentration of the emitter region is 10¹⁸ cm⁻³ or more, the breakdown voltage is determined by only a tunnel current. In a region where a breakdown voltage of about 3 V is required, the breakdown voltage is determined by mainly a tunnel current and can be approximated as follows:

    J.sub.t =(((2m*).sup.1/2 q.sup.3 εV)/(4π.sup.2 n.sup.2 E.sub.g.sup.1/2))×exp (-4(m*).sup.1/2 E.sub.g.sup.3/2)/(3qεn)!                          (1)

where

ε: electric field (V/m),

Eg: band gap

V: applied voltage

m*: effective mass

q: charge

h: (Planck constant)/2π

When equation (1) is applied to silicon, it yields the following equation.

    J.sub.t =(1.649×10.sup.4 εV)exp -3.052×10.sup.9 /ε!                                               (2)

In equation (2), if V=3 V and ε=1 MV/cm, then J_(t) =2.75×10⁻⁵ A/cm². If ε=1.5 MV/cm, then J_(C) =1.02 A/cm; and if ε=2 MV/cm, then J_(C) =2.33×10² A/cm². It is safe to determine an emitter impurity concentration with respect to that of the base region in an electric field ε=1 MV/cm when an emitter-base voltage is 3 V. That is, when the emitter size in an actual semiconductor device or the like can be reduced (e.g., 3×3 μm²), and an emitter junction depth is decreased (e.g., about 0.5 μm), a current near the emitter has an influence, and the current becomes larger than those given in equations (1) and (2).

A maximum electric field (stepwise junction approximation) ε_(m) of the p-n junction and a depletion layer width W can be represented by the following equations: ##EQU6## where

V_(bi) : diffusion voltage

ε_(s) : dielectric constant

N_(B) : impurity concentration of the base region

N_(D) : impurity concentration of the emitter region

V: applied voltage

The impurity concentration of the base region is plotted along the ordinate, and the impurity concentration of the emitter region is plotted along the abscissa in FIG. 15. In this case, results are obtained when ε=1 MV/cm and applied voltages V=1, 2, 3, 4, and 5 V.

When breakdown is to be determined by V=3 V and ε_(m) =1 MV/cm, a line of 3 V in. FIG. 15 is referred to. For example, when the impurity concentration (N_(B)) of the base region is 10¹⁸ cm⁻³, the impurity concentration (N_(D)) of the emitter region is preferably 4.5×10¹⁸ cm⁻³ or less.

If N_(B) =5×10¹⁸ cm⁻³ then N_(D) <1×10¹⁸ cm⁻³. Therefore, when N_(B) =1×10¹⁹ cm⁻³ then N_(D) <9×10¹⁷ cm⁻³.

When N_(B) is 1×10¹⁸ cm⁻³ or more, then N_(D) is set to be 4.5×10¹⁸ cm⁻³ or less. The concentration of 4.5×10¹⁸ cm⁻³ is a maximum limit.

When the applied voltage is set to be 2.5 V and the electric field ε=1 MV/cm is given, condition N_(D) <1×10¹⁹ cm⁻³ or less must be given to satisfy the above condition, as can be apparent from FIG. 11.

Judging from the above consideration, the concentration N_(D) was set to be 10¹⁹ or less.

The impurity concentration of the emitter region preferably has a lower limit of 10¹⁷ cm⁻³.

The preferable lower limit of the impurity concentration of the emitter region will be described below.

The collector current J_(C) is represented by equation (6) below:

    J.sub.B2 =(qn.sub.i.sup.2 D.sub.n /N.sub.B L.sub.n)cosech (W.sub.B /L.sub.n)× exp(V.sub.BE /kT)-1!                     (6)

If W_(B) <<L_(n), and V_(BE) >>kT, then equation (6) can be rewritten as follows:

    J.sub.C =(qn.sub.i.sup.2 D.sub.n)/(N.sub.B W.sub.B)·exp(V.sub.BE /kT)                                                      (7)

This equation is generally established within the following approximated range of a minority carrier:

    N.sub.D >>(n.sub.i.sup.2 /N.sub.B)·exp (V.sub.BE /kT)(8)

A boudary which does not satisfy the above condition is a current drive limit of this transistor.

The following equation in which (n_(i) ² /N_(B))·exp (V_(BE) /kT) is substituted by the impurity concentration N_(D) of the emitter region can be the current drive limit of the prescribed impurity concentration of the emitter region of the transistor:

    J.sub.C =q·(D.sub.n /W.sub.B)·N.sub.D    (b 9)

The collector current J_(C) of 10⁴ to 10⁵ A/cm² is generally required for any transistor. D_(n) =(kT/q)μ_(n) is used and conventional μn data is used to obtain W_(B) =0.05, 0.1, and 0.2 μm, thereby calculating the lower N_(D) limit, as shown in FIG. 16.

When JC therefore falls within the range of 10⁴ to 10⁵ A/cm², N_(D) is preferably 10¹⁷ or more.

An ohmic contact layer having an impurity concentration of 10¹⁹ to 10²¹ cm⁻³ may be formed on the surface to preferably reduce an ohmic resistance during formation of the emitter region.

2 B and Ge are simultaneously mixed to obtain an Si-Ge-B base, and a mixed crystal effect and a band-gap narrowing effect are simultaneously provided. The band gap of the base is reduced, and emitter injection efficiency can be improved. Therefore, the current gain bE can be increased. However, since the emitter impurity concentration is determined as described above, a decrease in base-emitter breakdown voltage and an increase in base-emitter path can be prevented.

The present invention will be described in more detail.

FIGS. 17A and 17 B are views showing band structures in semiconductors. Energy is plotted along the ordinate and a state density n(E) (i.e., the number of carriers per unit volume) is plotted along the abscissa in.each of FIGS. 17A and 17B.

FIG. 17A is a view showing a band structure of an n-type semiconductor having a low impurity density. In this case, an n-type donor level is separated from a conduction band. However, in a semiconductor containing a high impurity density, the width of a donor level is increased. The donor level becomes a donor band whose energy is coupled to that of the conduction band unique to the semiconductor. That is, as shown in FIG. 17B, a degenerate conduction band is formed. As a result, band-end tailing occurs, and the band gap is changed from E_(g) to E_(g) ', thus causing band narrowing of ΔE_(g) =E_(g) -E_(g).

Band narrowing of the n-type semiconductor has been described in FIG. 17. Similar band tailing occurs on the side of the valence band in a p-type semiconductor having a high impurity concentration, and narrowing of a band width. (prohibition band) E occurs. A band-narrowing value is approximated as follows:

    ΔE.sub.g =(3q.sup.2 /16πε.sub.s)×(q.sup.2 N/ε.sub.s kT).sup.1/2                             (10)

where q is a charge, e is a semiconductor dielectric constant, k is a Boltzmann constant, T is an absolute temperature, and N is an impurity density.

A band-narrowing value of Si at room temperature is given as follows:

    ΔE.sub.g =22.5(N/10.sup.18).sup.1/2  meV             (11)

If N=10¹⁸ cm⁻³, then ΔE_(g) =22.5 meV. An essential carrier density n_(i) '² upon occurrence of band-gap narrowing is given as follows:

    n.sub.i '.sup.2 =n.sub.i.sup.2 exp(ΔE.sub.g /kT)     (12)

where n_(i) ² is a carrier density without narrowing.

The analytically theoretical expressions of the base and collector currents are as follows (note that a portion in front of W_(E) of the emitter is assumed to contact a metal).

The base current consists of two components, i.e., a diffusion current of positive holes from the base to the emitter:

    J.sub.B1 =(qn.sub.i.sup.2 D.sub.p /N.sub.E L.sub.p)coth(W.sub.E /L.sub.p)× exp(V.sub.BE /kT)-1!                     (13)

and a recombination current of electrons injected from the emitter:

    J.sub.B2 =(qn.sub.i .sup.2 D.sub.n /N.sub.B L.sub.n)((cosh(W.sub.B /L.sub.n)-1)/(Sinh(W.sub.B L.sub.n))){exp(V.sub.BE kT)-1} (14)

A collector current is defined as follows:

    J.sub.C =(qn.sub.i.sup.2 D.sub.n /N.sub.B L.sub.n) cosech (W.sub.B /L.sub.n)× {exp(V.sub.BE /kT)-1}                    (6)

where q is a charge, n_(i) is an intrinsic semiconductor density, N_(E) is an emitter impurity density, N_(B) is a base impurity density, D is a positive hole diffusion coefficient, D_(n) is an electron diffusion coefficient, L_(p) is a positive hole diffusion length (approximated by D_(p) π_(p))1/2) L_(n) is an electron diffusion length (approximated by (D_(p) π_(p))^(1/2)) k is the Boltzmann constant, T is an absolute temperature, V_(BE) is a base-emitter forward-biasing voltage, π_(p) is a positive hole minority carrier lifetime, π_(n) an electron minority carrier lifetime, W_(E) is the thickness of the emitter, and W_(B) is the thickness of the base.

The emitter carrier density is presented by

    n.sub.iE.sup.2 =ni.sup.2  exp ΔE.sub.ge /kT!

and the base carrier density is represented by

    n.sub.iB.sup.2 =n.sub.i.sup.2  exp ΔE.sub.gb /kT!

When the above points are taken into consideration, terms n_(i) ² in equations (13), (14), and (6) are replaced with n_(iE) ², n_(iB) ², and n_(iB) ², espectively.

Since W_(B) <<L_(n) and J_(B1) >>J_(B2) are established, h_(FE) is approximated as ##EQU7## The current gain can be expressed when only band-gap narrowing is taken into consideration:

    h.sub.FE ≈h.sub.FE0 exp (ΔE.sub.gb -ΔE.sub.ge)/kT!(16)

where h_(FE) is a current gain without band-gap narrowing. The conventional BPT had an emitter impurity concentration N_(E) of about 10²⁰ to 10²¹ cm⁻³ and a base impurity concentration N_(B) of about 10¹⁶ to 10¹⁸ cm⁻³. For this reason, ΔE_(ge) is large, and ΔE_(gb) is almost zero. Therefore the current gain h_(FE) is smaller than the designed value.

FIG. 18 is a graph showing results of band-gap narrowing width calculations by using equation (2) when boron is doped in Si in a high concentration. The impurity density (cm⁻³) is plotted along the abscissa in FIG. 8, and the narrowed width ΔE_(g) (meV) of the band gap is plotted along the ordinate in FIG. 8. For example, when an emitter impurity concentration is 10¹⁸ cm⁻³ and a base impurity concentration is 10¹⁹ cm ⁻³, then ΔE_(g) =ΔE_(gb) -ΔE_(ge) ≈50 meV. Therefore, h_(FE) ≈7.4 h_(FE0).

The present invention has a great advantage in a base having a high impurity concentration. A p-type impurity has low solubility as compared with phosphorus (P) and arsenic (As) as n-type impurities.

FIG. 19 shows data of solubility of a solid-phase impurity in silicon. Temperatures (T °C.) are plotted along the abscissa, and the solubility in the solid phase is plotted along the ordinate. P-type impurities are exemplified by B, Ga, and Al. Of these impurities, B can be dissolved the highest concentration. When the impurity is doped to exceed the degree of solid solution, the impurity is precipitated in Si during the manufacturing process of the semiconductor device to form crystal defects, thereby adversely affecting the BPT characteristics. However, this depends on process temperatures.

Of the impurities, B can be most stably doped to a high concentration and is most suitable as a p-type impurity.

When an impurity is doped in a high concentration, a tetrahedral atomic radius of the impurity in Si poses a problem. More specifically, since Si has a diamond type crystal structure, it has tetrahedral bonds. In this case, an atomic radius E presents a problem. If a difference between atomic radii is given as 100×(r-r_(Si))r_(Si), the difference for B can be given as about -25. Since the atomic radius of B is smaller than that of Si, a stress occurs between a boundary between the B-nondoped emitter and collector having an impurity concentration of 10¹⁸ cm⁻³. When this stress is conspicuously increased, dislocation occurs. For this reason, electrical leakage occurs between the base-emitter path or the base-collector path, thereby decreasing a breakdown voltage. The current gain is also decreased.

According to the present invention, B and Ge are simultaneously doped in the base region.

A value of 100×(r_(GE) -r_(Si))/r_(Si) of Ge is almost +4. That is, the Ge value is larger than Si by 4%. Ideally, a ratio of the dose of Ge to that of Ge is set to be 8.25=25/4, crystal lattice distortion can be perfectly corrected.

According to the present invention, however, since the p-type base region is formed to have an impurity concentration distribution by, e.g., ion implantation, the semiconductor device has a structure in which lattice distortion tends not to occur. The Ge concentration is not limited to the one described above. The dose of B with respect to the base is continuously changed, and occurrence of defects and dislocation becomes difficult. Therefore, the defect occurrence limit of B has a peak concentration higher than the B concentration of 10¹⁸ cm⁻³.

The main purpose of doping Ge is to reduce a band gap smaller than that of Si upon doping of Ge into Si. The concentration which influences the band gap is not satisfied by 8.25 times that of B, but must be 10²⁰ cm⁻³ or more to obtain a satisfactory effect.

Ge has a band gap smaller than that of Si. The band gap of Si is E_(gSi) =1.1 eV, and the band gap of Ge is E_(gGe) ≈0.7 eV. When a crystal composition of Si_(1-x) Ge_(x) is obtained, its band gap can be approximated as follows:

    Eg'=E.sub.gSi -X(E.sub.gSi -E.sub.gGe)                     (17)

If X=0.1 then ΔE_(g) (=E_(g) '-E_(gSi)) is about 40 meV. At this time, the Ge concentration is about 5×10²¹ cm⁻³ and is 10 times or more the B concentration.

FIG. 20 is a graph showing an effect to a Ge band gap. A composition ratio X is plotted along the abscissa, and changes in band gap Eg, changes on the conduction band side ΔE_(c) and the valence band side ΔE_(v) in FIG. 20. As is apparent from FIG. 10, when B and Ge are simultaneously doped in the base, the base band gap can be reduced. Therefore, the current gain of the BPT can be increased.

Al, Ga, or the like may be used as a p-type impurity to be doped in the base. However, these p-type impurities have a value of 100×(r-r_(Si))/r_(Si) to be almost +8 and cannot correct lattice distortion of a substance having a small gap. That is, Si_(1-x) C_(x) has a property opposite to band-gap atomic radius. For example, although carbon (C) has a small atomic radius, Si_(1-x) C_(x) has a direction to increase a band narrowing. Therefore, the combination of Si and C is not preferable. B and Ge is a best combination for a p-type base.

In addition, when the base consists of an Si-Ge-B mixed crystal, injection of positive holes in the emitter can be suppressed, which does not a barrier for injection of electrons from the emitter to the base.

According to the present invention, since the oxide film formed between the emitter region and the base region is removed, characteristic variations between the BPTs can be eliminated.

According to the present invention, since the emitter region consists of single-crystal silicon, the current gain h_(FE) can be increased although any oxide film is not formed between the emitter region and the base region. The current gain h_(FE) can be increased due to the following reason.

When an oxide film is not formed, i.e., when a potential barrier is not present, as shown in FIG. 2, a diffusion current of positive holes from the base to the emitter can be given as follows:

    J.sub.B '=(q·n.sub.i.sup.2 ·D.sub.p /N.sub.E ·L.sub.p)×coth(W.sub.E /L.sub.P) exp(V.sub.BE /kT)-1!(1)

When the emitter region consists of single-crystal silicon, the diffusion length Lp of the minority carriers in the emitter region can be increased, and then J_(B) ' can be reduced. Therefore, the current gain can be further increased as follows:

    h.sub.FE =J.sub.C /(J.sub.B1 '+J.sub.B2)                   (4)

According to the present invention, V_(BE) dependency of the current gain h_(FE) can also be eliminated.

The current gain h_(FE) must not depend on V_(BE) under the condition exp(V_(BE) /kT)>>1.

When the relationship between h_(FE) and V_(BE) of the conventional DOPOS BPT (oxide film: 10 Å) is examined, the relationship shown in FIG. 22 is exhibited. When the thickness of the oxide film is further increased, a gradient (a portion in 1 in FIG. 22 of the graph is further increased.

The present inventors measured V_(BE) dependency of the base current J_(B) and the collector current J_(C) of the conventional BPT described above. J_(B) was almost proportional to exp(V_(BE) /kT), but J_(C) had a relation represented by J_(C) ∝exp(V_(BE) /nkT). The n value fell within the range of 1.01 to 1.05. That is, the n value was larger than the normal value by 1 to 5%. It seems that VBE dependency of h_(FE) is caused by J_(C). This J_(C) characteristic is expected such that the potential barrier on the conduction band side by an Si oxide film in the emitter region influences electron injection from the emitter to the base.

When a BPT is used as a sensor, V_(BE) dependency of h_(FE) greatly influences a ratio (γvalue) of an optical input to an electrical output. This dependency becomes a decisive problem when semiconductor devices are to be manufactured uniformly.

If the n value is given as n=1+δ(δ<<1), and

    J.sub.B ≈J.sub.B0 exp V.sub.BE /kT!                (5)

    J.sub.C ≈J.sub.C potential.sub.0 exp V.sub.BE/(1+δ)kT!(6)

then the current gain h_(FE) (=J_(C) /J_(B)) is obtained as follows: ##EQU8## That is, V_(BE) dependency of h_(FE) is thus found.

Since kT=0.025 at room temperature, assuming that δ=0.01 (i.e., n=0.01), δ/kT is 0.4 for VBE=1 V, thus changing dependency by 40%. Since VBE dependency of h_(FE) preferably falls within the range of 5%, condition δ≦1.25×10⁻³ must be satisfied.

To the contrary, according to the present invention, since the oxide film is removed to eliminate the potential barrier and minimize it as small as negligible. Therefore, V_(BE) dependency of h_(FE) can be eliminated or can be minimized as small as negligible.

The height of the potential barrier can be allowed to be about kT when, e.g, an n⁺ -type emitter is formed. However, when the height of the potential barrier exceeds kT, an influence on the electrical characteristics cannot be neglected. A potential barrier of a valence band naturally poses a problem in an p⁺ -type emitter.

When the width of the potential barrier is narrow, the carriers are conducted by a tunnel effect. In this case, the height of the potential barrier need not be kT or less.

A tunnel probability can be approximately (theoretically) represented as follows:

    P(E)∝exp{-A(.o slashed..sub.B.sup.3/2 /E)}          (8)

where E is an electric field (V/cm), .o slashed.B is a potential barrier height, and A is a constant defined as follows.: ##EQU9## where m is an effective mass e is a charge h is h/2π (h is the Planck constant. In case of silicon, since m* ≅0.3 m₀, e=1.602×10⁻¹⁹ C, and h=6.626×10⁻³⁴ J·sec, then A≈1.48×10⁷.

For example, if .o slashed._(B) =0.1 V and the potential barrier width is given as 10 Å, then P(E) ≅0.31. Since the tunnel current is given as J(E)∝E² P(E), a sufficiently large current can be obtained in a strong electric field E.

Since the oxide film in the emitter of the BPT is not uniformly formed, the potential barriers are not uniform in all regions. However, if the highest potential barrier is kT or less, no problem is posed.

According to the present invention, a conventional problem of a point defect or dislocation in a hetero interface between Si forming an emitter region and Si_(1-X) Ge_(X) forming a base region can be solved, thereby reducing a base current and increasing a current gain h_(FE).

Since the semiconductor device of the present invention has a high current gain as described above, it can increase a frequency f_(T). Therefore, when the semiconductor device of the present invention is adopted as a photoelectric transducer apparatus for use in a solid-state image pickup apparatus or the like, a read rate of the solid-state image. pickup apparatus can be improved.

The present invention will be described in detail below.

(Current Gain)

In the present invention, an emitter is made of Si, and a base is made of Si_(1-X) Ge_(X). Therefore, Si_(1-X) Ge_(X) is E_(g) '=E_(g) -ΔE_(g) on the basis of a band gap Eg of Si, i.e., the band gap is reduced by ΔE_(g). Analytically theoretical expressions of a base current and a collector current in a BPT of the present invention are as follows (note that the end of the emitter is brought into contact with a metal).

The base current mainly consists of two components, i.e., a diffusion current of positive holes from the base to the emitter:

    J.sub.Binj =(qn.sub.i.sup.2 D.sub.P /N.sub.E L.sub.P)×coth(W.sub.E/ L.sub.P) exp(V.sub.BE /kT)-1!                             (1)

and a recombination current of electrons injected from the emitter: ##EQU10## The collector current is given as follows: ##EQU11## where q; a charge

n_(i) ; an intrinsic semiconductor charge density (Si)

N_(E) ; an emitter impurity density

N_(B) ; a base impurity density

Dp; a positive hole diffusion coefficient

D_(N) ; an electron diffusion coefficient

L_(p) ; a positive hole diffusion length (≈(D_(p) π_(p))^(1/2))

L_(N) ; an electron diffusion length (≈(D_(N) π_(N))^(1/2))

k; a Boltzmann constant

T; an absolute temperature

V_(BE) ; a base-emitter forward biased electron

τ_(P) ; a minority carrier lifetime of the positive holes

τ_(N) ; a minority carrier lifetime of the electrons.

When ΔE_(g) /kT >>1,

    J.sub.Bing <<J.sub.GBrec

is obtained. When W_(B) <<L_(N), a current gain h_(FE) is represented by the following relation:

    h.sub.FE ≈2(L.sub.N /W.sub.B).sup.2                (4)

That is, in this case, ΔEg no longer has a large influence on the current gain.

Therefore, h_(FE) is uniquely determined by the value of (L_(N) /W_(B)) .

(Depth of Si_(1-X) Ge_(X))

The present invention is partly characterized in that the depth of Si_(1-X) Ge_(X) is the same as or larger than that of the base. This feature of the present invention will be described in detail below.

A case in which the Si_(1-X) Ge_(X) layer is shallower than the p-type layer will be described first.

In the present invention, since a layer consisting of Ge and a p-type impurity is formed by ion implantation, the Si_(1-X) Ge_(X) layer and the p-type layer do not always coincide with each other. When the Si_(1-X) Ge_(X) layer can be formed in only a region shallower than the p-type layer from the surface, a problem is posed.

When the Si_(1-X) Ge_(X) layer is shallower than the p-type layer, a potential of the BPT is obtained as shown in FIG. 11. Referring to FIG. 11, a forward bias is applied between the emitter and the base, and a reverse bias is applied between the base and the collector.

As shown in FIG. 11, when the Si_(1-X) Ge_(X) layer is formed halfway in the p-type region, a potential barrier of ΔE_(g) is formed in the base.

When the potential barrier is formed, an increase of:

    exp(ΔE.sub.g /kT)

in the collector current represented by equation (3) disappears. This is because minority carriers propagating in the base are interrupted by the potential barrier at a point A in FIG. 11. That is, carriers passing through the barrier become:

    exp(-ΔE.sub.g /kT)

Therefore, when the Si_(1-X) Ge_(X) layer is shallower than the p-type layer (base layer), the collector current is the same as that of a BPT of a normal homojunction.

Since an amount of minority carriers injected in the base is exp(ΔE_(g) /kT) times that obtained when the base is made of Si, the recombination current becomes larger than that of a homojunction. For this reson, the current gain h_(FE) of the BPT becomes smaller than that of the homojunction BPT.

For the reasons described above, the interface between Si_(1-X) Ge_(X) and Si must be formed in the depletion layer of the base and collector junction or in the collector. Alternatively, the interface must be formed in the depletion layer of the emitter and the base or at the emitter side. In this manner, diffusion of the minority carriers injected from the base to the emitter can be interrupted by the barrier at the hetero interface. That is, the hetero interface between Si and Si_(1-X) Ge_(X) must not be formed at a neutral region of a region serving as a base.

FIG. 11A is a schematic drawing showing an energy band diagram as an example of an SiGe region extending over a base region.

(Inclined Heterojunction)

The reference of the thickness of a transition region may be determined by a value obtained from FIG. 23. For example, if a mixed crystal ratio X=0.3, a peak concentration width may be 300 Å or less; if X=0.2, it may be 500 Å or less; and if X=0.1, it may be 1,500 Å or less. By analyzing an impurity distribution upon ion implantation by, e.g., an SIMS (secondary ion mass analyzer), ion implantation conditions can be easily determined.

FIG. 24 shows an analysis example obtained by the SIMS. Comparison is performed with respect to the SIMS analysis result in accordance with a designed mixed crystal ratio X, and the thickness of the heterojunction, i.e., the ion implantation conditions are determined in accordance with data shown in FIG. 23.

The mixed crystal ratio X is a ratio between numbers of Si and Ge atoms Since Si is substantially 5×10²² cm⁻³ The ratio X can be easily calculated by a dose of Ge. That is, X is preferably 0.5 or less.

In the present invention, since the emitter is made of silicon and the base is made of Si_(1-X) Ge_(X) is E_(g) '=E_(g) -ΔE_(g) on the basis of a band gap E_(g) of Si. That is, the band gap is narrowed by ΔE_(g).

(Cutoff Frequency f_(T) of BPT)

Generally, f_(T) is analytically represented by the following equation:

    1/2πf.sub.T =(kT/qI.sub.C +r.sub.EE)(C.sub.be +C.sub.bc)+τ.sub.B +τ.sub.C +r.sub.CC C.sub.bc                           (5)

where I_(C) is a collector current, C_(be) is a base-emitter capacitance, C_(bc) is a base-collector capacitance, r_(EE) is an emitter series resistance, r_(CC) is a collector series resistance, τ_(B) is a base travel time, τ_(C) is a travel time in a collector depletion layer.

In order to increase f_(T), the right-hand side of equation (5) must be reduced.

The resistance r_(EE) is mostly caused by an ohmic contact and depends of a surface concentration and an electrode. That is, the surface need only be N⁺⁺.

r_(CC) is influenced more by the thickness of an n⁻ -type region 3 than those of n+-type regions denoted by reference numerals 2 and 7 shown in FIG. 25.

The physical meaning of f_(T) is a time delay of carriers from the emitter to the collector. f_(T) is represented by using τ_(ec) as follows:

    f.sub.T= 1/(2πτ.sub.ec)                             (6)

τ_(ec) is relatively shown in FIG. 26. τ_(ec) is reduced together with I_(C) and increased at a high rate from a minimum point. A inflection point determined by J₁ is a point on which a high-current effect has an influence.

For example, if f_(T) =10 GHz, τ_(ec) =16 psec.

The following description will be based on these values.

1 τ_(B) is a base travel time and represented by the following equation:

    τ.sub.B =W.sub.B.sup.2 /ηD.sub.B                   (7)

If the base is uniformly doped, n =2. If a potential gradient ε_(bi) is present, D is represented by the following equation:

    η≈2 1+(ε.sub.bi /ε.sub.0).sup.3/2 !(8)

where

    ε.sub.0 =2D.sub.B /μ.sub.B W.sub.B

By using an Einstein relation,

    ε.sub.0 =kT/qW.sub.B

is obtained, and the potential gradient is obtained as follows:

    ε.sub.bi =ΔV/W.sub.B

By substituting these equations into equation (7), the following relation is obtained:

    η=2 1+(qΔV/2kt).sup.3/2 !                        (9)

Therefore, if ΔV=2kT/q, η=4, and τ_(B) is 1/2. Assuming that

N_(B=) 10¹⁸ cm⁻³

μn=300 cm⁻² /sec·V,

and

W_(B) =0.1 μm,

W_(B) /2D_(B) =13 psec

is obtained. In this case, if

ΔV=2kT/q,

W_(B) /2D_(B) =6.5 psec

is obtained. If

ΔV=4kT/q,

W_(B/) 2D_(B) =3.4 psec is obtained. That is, if ΔV=2kT/q or more, a drift effect is obtained.

This effect is enhanced at a low temperature. For example, since a liquid nitrogen temperature of 77° K is about 1/4 of a room temperature (300° K),

    ΔV=4kT/q=0.026e

is obtained, and a small potential gradient has a very large effect.

2 is a transit velocity in a collector depletion layer and represented by the following equation:

    τ.sub.VC W.sub.BC /2V.sub.s                            (10)

where WBC is a base-collector depletion layer width and V_(s) is a transit velocity.

V_(E) is normally a saturation velocity of 107 cm/sec (≦10⁵ V/m). Therefore, if W_(BC) =0.1 μm, 0.5 μm, and 1.0 μm, τ_(VC) is 0.5 psec, 2.5 psec, and 5.0 psec, respectively. Base and collector concentrations may be determined such that W_(BC) becomes 1 μm or less.

3 τ_(e) (emitter charge time) will be described below.

Normally, if C_(be) >>C_(bc) and an emitter area A_(E) =1.5×1.5 μm², a high current up to J_(C) =1×10⁴ to 10⁵ A/cm² is used. Therefore,

    I.sub.C =J.sub.C ·A.sub.E =0.225 to 2.25 mA

is obtained, and kT/qI_(C) at room temperature is obtained as follows:

    kT/qI.sub.C =111 to 11.1 Ω

In this case, ##EQU12## Therefore,

    kT/qI.sub.C >>r.sub.EE

is obtained since J_(C) =10⁴ A/cm². Therefore assuming that τ_(E) <5 psec,

    (kT/qI.sub.C)C.sub.be <5 psec

is obtained, and ##EQU13## is obtained. That is, if C_(be) is 45 fF or less, τ_(e) is 5 psec or less. For example, if C_(be) is 10 fF, τ_(e) is 1 psec.

If C_(be) is 10 fF or less, the speed of τ_(e) is not problematic at all.

4 A collector charge time τ_(c) is represented by the following equation:

    τ.sub.C =r.sub.CC ·εS /W.sub.BC       (12)

Assuming that S=4×4 μm², since

    τ.sub.c =r.sub.CC 1.66×10.sup.6 /W.sub.BC

is obtained,

    r.sub.CC <6×10.sup.6 W.sub.BC

is obtained. Therefore, even if W_(BC) =0.1 μm, no problem is posed when r_(CC) <60 Ω or less.

(Impurity Concentration)

(1) Emitter Impurity Concentration

The lower limit of an emitter impurity concentration will be described below.

As described above, the collector current is represented by equation (3). If W_(B) <<L_(n) and V_(BE) >>kT, the following equation is obtained:

    J.sub.C =(qn.sub.i.sup.2 D.sub.n /N.sub.B W.sub.E)exp{(V.sub.BE +ΔE.sub.g)/kT}(13)

Normally, this equation is satisfied when the following equation of minority carrier approximation is satisfied:

    N.sub.D >>(n.sub.i.sup.2 /N.sub.B){(V.sub.BE +ΔE.sub.g)/kT}(14)

A region in which the above equation is not satisfied corresponds to a transistor current drive limit.

Therefore,

    J.sub.C ≈q(D.sub.n /W.sub.B)N.sub.D                (15)

obtained by substituting

    (n.sub.i.sup.2 /N.sub.B){(V.sub.BE +ΔE.sub.g)/kt}

of equation (13) by an emitter concentration N_(D) is a current drive limit for emitter concentration definition of this transistor.

A transistor normally requires J_(C) of 10⁴ to 10⁵ A/cm². Since

    D.sub.n =(kT/q)μn,

the lower limit of N_(D) is calculated as shown in FIG. 27B by using data of μn shown in FIG. 27A and assuming that W_(B) =0.1. In this case, J_(C) =1×10⁴, 5×10⁴, and 1×10⁵ A.

(2) Collector Impurity Concentration

A current limit determined by a collector impurity density is represented by the following equation:

    J.sub.C =q·n·V.sub.s                     (16)

In this case, a base-collector depletion layer transits a saturated velocity of

    V.sub.s ≈10.sup.7 cm/sec,

and n has a limit of N_(C). For example, if J_(C) is 1×10⁴ A/cm², N_(C) is 6.2×10¹⁵ cm⁻³. If J_(C) is 1×10⁵ A/cm², N_(C) is 6.2×10¹⁶ cm⁻³. A current represented by J₁ in FIG. 26 normally has this value. When a current having a value determined by J₁ =J_(cmax) or more flows, carriers are injected from the base to cause a base spreading effect, and τ_(B) represented by equation (7) increases at a high rate. Therefore, as shown in FIG. 26, a phenomenon in which τ_(ec) increases at a high rate appears. If N_(C) is 3×10¹⁶ cm⁻³, τ_(ec) does not increase at a high rate until 5×10⁴ A/cm².

(3) Base Impurity Concentration

A base impurity concentration will be described below.

A base carrier concentration is good as long as the number of injected minority carriers is

    (n.sub.i.sup.2 /NB)exp{(V.sub.BE +ΔE.sub.g)/kT}

or more. If the number is larger than this value, neutrality of a charge can be maintained. That is, assuming that a base impurity concentration is N_(B), the following relation need only be satisfied:

    N.sub.B >>(n.sub.i.sup.2 /N.sub.B) exp{(V.sub.BE +ΔE.sub.g)/kT}(17)

When N_(B) becomes close to n_(i) exp{(V_(BE) +ΔE_(g))/ 2kT}, a space-charge effect appears, and a resistance in the base rises at a high rate.

Therefore, if N_(B) ≦ND, a current drive capability determined by the emitter impurity density can be assured.

Base and collector impurity densities will be examined on the basis of the width WBC of the base-collector depletion layer. W_(BC) and a width X_(p) of a depletion layer spreading in the base are.represented by: ##EQU14## and,

    X.sub.P =N.sub.C /(N.sub.B +N.sub.C)                       (19)

FIG. 28 shows a relationship of depletion layer thicknesses W_(BC) and X_(p) (μm) with respect to the base impurity density N_(B) obtained under predetermined conditions of ΔEg=0.1 eV, room temperature, and V_(R) =5 V, and by using connector impurities densities N_(C) =1×10¹⁶, 3×10¹⁶ and 6×10¹⁶ cm⁻³ as a parameter.

When N_(C) =3×10¹⁶ cm⁻³, W_(BC) =0.35 μm, and τ_(VC) of equation (10) is 1.75 psec, thereby posing no problem. τ_(B) obtained from equation (7) indicates that at least W_(B) must be 0.1 μm or less. When 5 V is applied across the base and the collector, X_(p) is at least X_(p) <W_(B). In addition, as is apparent from FIG. 28, N_(B) must be about 2×10¹⁷ cm⁻³ or more.

As described above, in the hetero BPT of the present invention, if the current driving condition of J_(C) =5×104 A/cm,

1 the collector impurity density must satisfy:

    N.sub.C ≦3×10.sup.16 cm.sup.-3

2 the base impurity density must satisfy:

    N.sub.B ≦2×10.sup.17 cm.sup.-3

note that the base width W_(B) ≦0.1 μm, and

3 the emitter impurity density must satisfy:

    N.sub.D ≧2×10.sup.17 cm.sup.-3

Assuming that the emitter size is A_(E) =1.5×1.5 μm , the base area is A_(B) 4×4 μm² the emitter-base voltage is a bias value, and the base-collector voltage =+5 V, C_(be) =3.6 fF, C_(bc) =4.4 fF, and W_(BC) =0.39 μm. Therefore, and τ_(c) are sufficiently small. τ_(e) is basically in inverse proportion to I_(C) and is vary small in a high-current region. Therefore, othe lower limit of τ_(ec) is determined by the base travel time τ_(B).

In a conventional BPT, τ_(B) determines the upper limit of f_(T). At a low temperature, D_(B) is further reduced on the basis of a relationship of D_(B) =(kT/qK)μn.

To the contrary, the hetero BPT of the present invention having a potential gradient can keep its high performance.

(Charge Storage)

Charge storage in the hetero BPT of the present invention will be described below.

In a charge storage device such as a BPT, a propagation delay time T can be approximated by the following equation:

    T=Q.sub.BPT/.sup.I C                                       (20)

where Q_(BPT) is a charge stored in a BPT and I_(C) is a collector current. In order to reduce T, Q_(BPT) is reduced and I_(C) is increased.

In the hetero BPT according to the present. invention, I_(C) is increased by exp (ΔE_(g) /kT) as compared with that of a conventional Si BPT. Therefore, T can be reduced by only this increase. A better effect, however, can be obtained by reducing Q_(BPT).

A stored charge QBPT of the BPT can be approximated by a charge stored in the base as follows:

    Q.sub.BPT q∫(n-n.sub.p)dx                             (21)

When the base impurity density is constant and no band gap gradient is present, the following relation is obtained: ##EQU15## Therefore,

    T=(1/2)(W.sub.B.sup. 2/D.sub.n)                            (23)

is obtained. This is the same as τ_(B) in equation (7).

An effect of increasing a current by exp(ΔE_(g) /kT) becomes the same as that of a conventional homo BPT if a stored charge increases.

In order to reduce the stored charge Q_(BPT) a potential gradient must be formed in the base to obtain a drift.

Referring to FIG. 29, the abscissa represents a distance standardized by a base width W_(B), and the ordinate represents a relative carrier number n obtained by normalizing injected carriers by a carrier number n (0) at an emitter-base junction (x=0) in a BPT in which an impurity density is constant in the base.

When a potential distribution is provided in the base and made steep as indicated by A→B→C in FIG. 10, the carrier number is reduced. If ΔV=2kT/q is set in order to halve τ_(B), the stored charge is halved.

A diffusion capacitance CDE equivalently representing storage of minority carriers is given by the following equation:

    C.sub.DE =dQ.sub.BPT /dV.sub.BE =q·-∫ (n-n.sub.p)dx!(24)

If a base impurity is constant and no potential gradient is present CDE is given as follows: ##EQU16## C_(DE) is reduced from this value by a potential gradient. Since C_(DE) is substantially proportional to a stored charge. amount, C_(DE) is halved if ΔV=2 kT/q.

That is, performance of the HBT can be improved by a potential gradient (ε_(bi) =ΔV/W_(B)).

(Photoelectric Transducer Apparatus)

According to the present invention, f_(T) of a photoelectric transducer apparatus can be improved, and a nondestructive degree can be improved by increasing a current gain.

Therefore, there is provided a solid-state image pickup apparatus capable of reading data from the same element a plurality of times without performing correction and capable of performing high-speed reading.

According to the present invention, there is provided a semiconductor device having small characteristic variations, a high current gain h_(FE) , and a high frequency, and a photoelectric transducer apparatus. using the same.

1 Since the oxide film formed at the interface between the base and the emitter of the conventional semiconductor device is removed, characteristic variations can be minimized. In addition, the current gain h_(FE) is caused not to depend on V_(BE).

2 Since the emitter region consists of a single crystal, the diffusion length of the minority carriers (positive holes) can be increased as compared with the device in which an emitter region consists of polysilicon. Therefore, the diffusion current of the positive holes from the base to the emitter can be increased, and the current gain h_(FE) can be increased.

3 Since the carrier concentration is set to be 1×10¹⁹ cm⁻³ or less near the interface between the emitter region and the base region in the emitter region, band-gap narrowing can be prevented.

4 Since an emitter region having a high impurity concentration of 1×10¹⁹ to 1×10²¹ cm⁻³ is formed between the emitter electrode and the emitter region, the positive holes are blocked, and the ohmic resistance can be reduced.

5 Since B and Ge as impurities are doped in the base region, both the mixed crystal effect and the band-gap narrowing effect can be obtained. Therefore, the base band gap can be reduced, and at the same time emitter injection efficiency can be improved.

6 Since the photoelectric transducer apparatus using the above BPT, a photoelectric transducer apparatus having a high frequency can be provided.

The present invention will be described in detail below.

(Emitter Region)

In the BPT of the present invention, as indicated by items 1 and 2, the oxide film formed at the interface between the base and the emitter can be eliminated, and the emitter region consists of single-crystal silicon.

As described above, since the oxide film is formed at the interface between the base and the emitter in the conventional BPT, the current gain h_(FE) can be increased. In equation (1), when the thickness W_(E) of the emitter is decreased, J_(B1), is reduced. Therefore, h_(FE) is represented as follows:

    h.sub.FE =J.sub.C /(J.sub.B1 +J.sub.B2)                    (9)

When the oxide film is formed at the interface between the base and the emitter, a diffusion current of positive holes from the base to the emitter is defined as follows:

    J.sub.B1 ={(q·n.sub.i.sup.2 ·D.sub.p)/(N.sub.E ·L.sub.p)}×tanh(W.sub.E /L.sub.p){exp(V.sub.BF /kT)-1}(10)

When the oxide film is eliminated, the diffusion current of positive holes from the base to the emitter is represented by equation (1). The diffusion length L_(p) of the positive holes as minority carriers can be reduced, and J_(B1) is increased accordingly. Therefore, the current gain h_(FE) is lowered.

In order to solve the above problem, the emitter consists of single-crystal silicon according to the present invention. When the emitter consists of single-crystal silicon, the diffusion length L of positive holes serving as minority carriers in the emitter is increased. Therefore, the current gain bE is increased as compared with the case wherein the emitter consists of polysilicon without an oxide film.

Since the oxide film is eliminated, the current gain h_(FE) does not depend on VBE (only under the condition exp(V_(BE) /kT)>>1).

FIG. 30 is a graph illustrating VBE dependency of h_(FE). The current gain h_(FE) is plotted along the ordinate, and the voltage V_(BE) is plotted along the abscissa in FIG. 30. A region 1 is a current region generally represented by equations (1) to (3) and (9). A region

2is a large-current region which cannot be represented by equations (1) to (3) and (9).

When a silicon oxide film in a conventional DOPOS BPT is about 10 Å, the current gain h_(FE) depends on V_(BE) in the region 1, as indicated by a line (b) in FIG. 30. When the thickness of SiO₂ is increased, this dependency is increased, as indicated by a line (a) in FIG. 30. To the contrary, when the oxide film is eliminated, V_(BE) dependency of h_(FE) is decreased as small as negligible, as indicated by a line (c) in FIG. 30. A cause of VBE dependency of h_(FE) is taken into consideration by independently examining J_(B) and J_(C) components. A base current J_(B) is almost proportional to exp(V_(BE) /kT), but a collector current J_(C) is represented by JC∝exp(V_(BE) /nkT). The n value was about 1.01 to 1.05 for a thin silicon oxide film (10 to 30 Å) and was larger than a normal value by about 1 to 5%. For this reason, the current gain h_(FE) depends on V_(BE). That is, this is the reason why V_(BE) dependency of the current gain h_(FE) can be eliminated by removing the thin silicon oxide film. The n value is large in the presence of the thin silicon oxide film because the potential barrier on the conduction band by the silicon oxide film seems to influence injection of electrons from the emitter to the base.

If the n value is defined as n=1 δ(δ<<1), and

    J.sub.B J.sub.B0 ·exp(V.sub.BE /kt)               (11)

    J.sub.C ≈J.sub.C0 exp{V.sub.BE /(1+δ)kT }    (12)

the current gain h_(FE) =JC/JB can be obtained as follows: ##EQU17## Therefore, VBE dependency of h_(FE) can be obtained. Since kT=0.025 V in room temperature, and if δ=0.01 (n=0.01) and V_(BE) =1 V are given, h_(FE) =0.6h_(FE0). The current gain can be increased by 40% as compared with V_(BE) =0 V. In order tO cause the change in h_(FE) at V_(BE) =1 V to fall within 5%, condition δ≦1.25×10⁻³ must be established. In this manner, the emitter potential barrier greatly influences the h_(FE) characteristics of the BPT.

When a BPT is used as a sensor, V_(BE) dependency of h_(FE) greatly influences an optical input vs. electrical output ratio (γvalue). This dependency also presents a significant problem when elements are to be uniformly manufactured.

A maximum height of this potential barrier can be allowed to about kT in a conduction band of an n -type emitter. When the maximum height exceeds kT, electrical characteristics are adversely affected. The potential barrier of the valence band presents a significant problem in a p⁺ -type emitter.

Fermi levels of semiconductors can be approximated as follows:

    n-type semiconductor E.sub.F -E.sub.i =kT·1.sub.n (N.sub.D /n.sub.i)                                                 (14)

    p-type semiconductor E.sub.i -E.sub.F =kT·1.sub.n (N.sub.A /n.sub.i)                                                 (15)

where E_(F) is a Fermi level, N is an n-type impurity density and N_(A) is a p-type impurity density.

Equations (14) and (15) can be established within the range of Boltzmann,statistics. When an impurity density is high enough to be measured by Fermi-Dirac statistics, equations (14) and (15) cannot be used.

FIG. 31 shows a relationship between the Fermi level E_(F) and the carrier density in Si. Energy normalized with kT is plotted along the abscissa in FIG. 31. The normalized energy is (E_(F) -E_(C))/kT for an n-type semiconductor and (E_(V) -E_(F))/kT for a p-type semiconductor. E_(C) and E_(V) are energies of the conduction and valence bands. The carrier density (cm⁻³) is plotted along the ordinate in FIG. 31 and is almost equal to the impurity density at room temperature. As is apparent from FIG. 31, when the normalized energy is zero, the Fermi level coincides with E_(C) and E_(V). However, when the concentration is higher, E_(F) is not present in the prohibition band but is present in band band. In this case, the impurity concentration is 2.1×10¹⁹ cm⁻³ for n-type, or 8×10¹⁸ cm⁻³ for p-type.

The alternate long and short dashed line in FIG. 3 exhibits a case obtained by using the Boltzmann statistics. When the normalized energy is higher than -1, a difference between the Boltzmann statistics and the Fermi-Dirac statistics occurs. In this case, unless the Fermi-Dirac statistics is used, Fermi energy cannot be accurately evaluated.

The difference between the n- and p-type semiconductors is caused by different band structures and different effective masses. This value varies depending on the types of material.

(Region Having High Impurity Concentration)

According to the present invention, a region having a high impurity concentration of 10¹⁹ cm⁻³ or more is formed on the emitter surface. In this case, the potential distribution is shown in FIG. 32, and a potential barrier ΔE_(B) is formed in the emitter, as is apparent from FIG. 32. This potential barrier is formed by a Fermi level difference with respect to the carrier concentration, as shown in FIG. 32. The positive holes injected from the base are blocked by this potential barrier. Therefore, the base current is reduced, and the current gain h_(FE) can be increased.

In order to obtain a sufficient positive hole blocking effect, the impurity concentration is set to satisfy condition ΔE_(B) >kT. If condition ΔE_(B) >2kT is satisfied, this effect is further enhanced. A current by recombination of positive holes injected from the base can be represented by equation (1), i.e., the same equation as obtained when the thin silicon oxide film is formed between the base and the emitter. In this case, if W_(E) '<<L_(p), then J_(B1) is decreased, and the h_(FE) of the BPT can be increased.

If an impurity concentration within the emitter near the base is given as N_(E1), and an impurity concentration of the region having a high impurity concentration is given as N_(E2), inequality ΔE_(B) <kT is derived from equation (14) under the following condition:

    N.sub.E2 >eN.sub.E1                                        (16)

More accurately, N_(E1) and N_(E2) can be obtained by using the data shown in FIG. 3. Condition ΔE_(B) ≧kT is equal to condition N_(E2) ≧eN_(E1). W_(E) ' must satisfy condition 2W_(E) '≧L_(p) so as to obtain a satisfactory effect.

(Base Region)

According to the present invention, since the impurity concentration of the base is increased, the current gain h_(FE) can be increased.

For example, in equation (8), if the emitter impurity density is given as 10¹⁸ cm⁻³ and the base impurity density is given as 10¹⁹ cm⁻³ then ΔE_(gb) -ΔE_(ge) ≈50 meV Therefore, h_(FE) ≈74h_(FE) can be obtained.

B is preferably used as a base impurity.

A p-type impurity has low solubility as compared with phosphorus (P) and arsenic (As) as n-type impurities. FIG. 19 shows data of solubility of a solid-phase impurity in silicon. Temperatures (T ° C.) are plotted along the abscissa, and the solubility in the solid phase is plotted along,the ordinate. P-type impurities are exemplified by B, Ga, and Al. Of these impurities, B can be dissolved in the highest concentration. When the impurity is doped to exceed the degree of solid solution, the impurity is precipitate in Si during the manufacturing process of the semiconductor device to form crystal defects, thereby adversely affecting the BPT characteristics (note that this depends on process temperatures).

By the above reason, B can be most stably doped up to a high concentration and is most suitable as a p-type impurity.

According to the present invention, Ge together with B is doped in the base. The purpose of Ge doping is to narrow the band gap.

Ge has a band gap smaller than that of Si. The Si band gap is E_(gSi) ≈1.1 eV, whereas the Ge band gap is E_(gGe) ≈0.7 eV. When a crystal composition is given as Si_(1-x) Ge_(x), its band gap can be approximated as follows:

    E.sub.g '=E.sub.gSi -X(E.sub.gSi -E.sub.gGe)               (17)

If X 0.1, therefore, ΔE_(g) (=E_(g) '-E_(gSi)) is about 20 meV .

The limitation of the Ge concentration will be described below.

According to the present invention, the Ge concentration is preferably 8.25 times or more the B concentration due to the following reason.

When an impurity is doped in a high concentration, a tetrahedral atomic radius of the impurity in Si poses a problem. More specifically, since Si has a diamond type crystal structure, it has tetrahedral bonds. In this case, an atomic radius E presents a problem. If a difference between atomic radii is given as 100×(r_(Ge) -r_(Si))/r_(Si), the difference for B can be given as about -25.

The atomic radius of Ge, i.e., 100×(r_(Ge) -r_(Si))/r_(Si), is about +4. That is, the Ge value is larger than Si by 4%. Ideally, a ratio of the dose of Ge to that of Ge is set to be 8.25=25/4, crystal lattice distortion can be perfectly corrected.

According to the present invention, however, since the p-type base region is formed to have an impurity concentration distribution by, e.g., ion implantation, the semiconductor device has a structure in which lattice distortion tends not to occur. However, the Ge concentration is not limited to the one described above. The Ge dose is preferably 8.25 times or more the B dose.

Al, Ga, or the like may be used as a p-type impurity to be doped in the base (however, these p-type impurities have 100×(r-r_(Si))/r_(Si) ≈+8) and cannot correct lattice distortion of a substance having a small atomic radius. For example, although carbon (C) has a small atomic radius, Si_(1-x) C_(x) has a direction to increase a band gap. That is, Si_(1-x) C_(x) has a property opposite to band-gap narrowing. Therefore, the combination of Si and C is not preferable. B and Ge is a best combination for a p-type base.

The lower limit value of the base concentration is determined by band tailing and is therefore given as 1×10¹⁸ cm⁻³. On the other hand, the upper limit of the base concentration is about 1×10²⁰ since the solid solution limit shown in FIG. 19 is given.

(Emitter Concentration)

When a base concentration is increased, a base-emitter junction breakdown voltage is decreased. When a semiconductor device is used in a normal IC, a minimum breakdown voltage is 2 V or more. However, when a sufficient margin is to be taken into consideration, a minimum breakdown voltage is preferably 3 V.

A p-n junction breakdown voltage is determined by avalanche amplification when the impurity concentration of an emitter region is 10¹⁷ cm⁻³ or less. However, when the impurity concentration of the emitter region is 10¹⁷ cm⁻³ or more, an influence of a tunnel current is enhanced. When the impurity concentration of the emitter region is 10¹⁸ cm⁻³ or more, the breakdown voltage is determined by only a tunnel current. In a region where a breakdown voltage of about 3 V is required, the breakdown voltage is determined by mainly a tunnel current and can be approximated as follows:

    J.sub.t ={(2m*).sup.1/2 q.sup.3 ε·V}/{4·π.sup.2 ·h·E.sub.g.sup.1/2 }×exp{(-4·m*,.sup.1/2 ·E.sub.g.sup.3/2)/(3·q·ε·h)}(18)

where is an electric field (V/m), Eg is a band gap, V is an applied voltage, m* is an effective mass, q is a charge, h is (Planck constant)/2 π. When equation (18) is applied to silicon, it yields the following equation.

    J.sub.t 1.649×10.sup.4 ·ε·V·exp -3.052×10.sup.9 /ε!                                               (19)

In equation (19), if V=3 V and ε=1 MV/cm, then J_(t) =2.75×10⁻⁵ A/cm². If ε=1.5 MV/cm, then J_(t) =1.02 A/cm; and if ε=2 MV/cm, then J_(t) =2.33×102 A/cm².

In an integrated circuit, a base-emitter breakdown voltage is required to be at least 2 V. For a safe margin, an impurity concentration is preferably higher than a lower limit of the emitter impurity concentration obtained when a voltage applied to the emitter-base path is set to be 3 V and an electric field is given as ε=1 MV/cm due to the following reason. In a practical semiconductor device or the like, when an emitter size is reduced (e.g., 3×3 μm²) and the emitter junction becomes shallow (0.5 μm or less), a current becomes larger than those given by equations (18) and (11) by an influence of currents near the emitter.

The lower limit of the emitter impurity concentration will be described below.

As described above, the collector current is represented by equation (3) but is given under the condition W_(B) <<L_(n) and V_(BE) >>kT as follows:

    J.sub.C ={(q·n.sub.i.sup.2 ·D.sub.n)/(N.sub.B ·W.sub.B)}exp(V.sub.BE /kT)                      (20)

This equation is established within the range approximated by the minority carriers as follows:

    N.sub.D >>(n.sub.i.sup.2 /N.sub.B)exp(V.sub.BE /kT)        (21)

(N_(D) is the emitter impurity concentration)

The limit at which the above condition becomes unsatisfied is a current drive limit of the transistor.

The current drive limit of the prescribed emitter impurity concentration of this transistor is given by substituting (n_(i) ² /N_(B))exp(V_(BE) /kT) with the emitter concentration N_(D) as follows:

    J.sub.C ≈q·(D.sub.n /W.sub.B)·N.sub.D(22)(22)

The normal transistor requires J_(C) of 1×10⁴ to 10⁵ A/cm².

Since D_(n) =(kT/q)μ_(n), then μ_(n) is used as conventional data to obtain D_(n). The lower limit of N_(D) is calculated using W_(B) =0.05, 0.1, and 0.2 μm, and the calculation results are obtained, as shown in FIG. 16A.

That is, the upper limit N_(D) concentration for V=6 V and ε_(m) =1 MV/m of FIG. 15 and the lower limit N_(D) concentration of FIG. 16 are solely determined by the W_(B) values. FIG. 33 shows data obtained by using J_(C) as a parameter at W_(B) =0.1 μm.

In an integrated circuit, a base-emitter breakdown voltage is required to be at least 2 V. For a safe margin, an impurity concentration is preferably higher than a lower limit of the emitter impurity concentration obtained when a voltage applied to the emitter-base path is set to be 3 V and an electric field is given as ε=1 MV/cm due to the following reason. In a practical semiconductor device or the like, when an emitter size is reduced (e.g., 3×3 μm²) and the emitter junction becomes shallow (0.5 μm or less), a current becomes larger due to an influence of currents near the emitter.

The upper limit of the emitter impurity concentration will be described below.

A maximum electric field (stepwise junction approximation) em of a p-n junction and a depletion layer width W can be represented as follows:

    ε.sub.m = {2q(V.sub.Bi +V)/ε.sub.s }{(N.sub.B ·N.sub.D)/(N.sub.B +N.sub.D)}!.sup.1/2           (23)

    W= {2ε.sub.s/q }{(N.sub.B ·N.sub.D)/(N.sub.B +N.sub.D)}{V.sub.bi +V}!.sup.1/2                          (24)

where V_(bi) is a diffusion potential, ε_(s) is a dielectric constant, N_(B) is a base concentration, N_(D) is an emitter concentration, and V is an applied voltage.

FIG. 15 is a graph showing a relationship between the base concentration N_(B) and the emitter concentration N_(D) at ε_(m) =1 MV/cm when applied voltages V are 1, 2, 3, 4, and 5 V. For example, when an applied voltage is 3 V or more, the emitter concentration is set to be 4.5×10¹⁸ cm⁻³ or less for the base concentration of 1×10¹⁸ cm⁻³. When the base concentration is given as 5×10¹⁸ cm⁻³ the emitter concentration is set to be 1×10¹⁸ cm⁻³ or less.

When the base concentration is given as 1×10¹⁹ cm⁻³, then the emitter concentration is set to be 9×10¹⁷ cm⁻³ or less. When the base concentration is given as 1×10¹⁸ cm⁻³ or more, the emitter concentration is set to be 4.5×10¹⁸ cm⁻³.

When the applied voltage is 2.5 V and ε_(m) =1 MV/cm is or less. This emitter concentration is the upper limit. given, the emitter concentration is set to be 1×10¹⁹ cm⁻³. This emitter concentration is the upper limit.

(Collector Impurity Concentration)

When the base concentration is increased, as described above, a base-collector breakdown voltage for the collector poses a problem.

The upper limit of the collector concentration is determined by the above breakdown voltage. A breakdown voltage in the emitter-collector (E-C) path of the BPT is determined by a base-collector (B-C) breakdown voltage when the base concentration is increased. In a BPT operated at 5 V, the E-C breakdown voltage is required to be 10 V for a safe margin. In order to assure the breakdown voltage of 10 V or more, the collector concentration is set to be N_(C) <1.5×10¹⁷ cm⁻³ In 3-V driving the breakdown voltage is required to be 6 V or more. Therefore, the collector impurity concentration N_(C) is about 3×10¹⁷ cm⁻³.

The lower limit of the collector concentration is determined by the current limit. On the collector side, an acceleration electric field is applied to the base-collector junction. In a high electric field (10⁵ V/cm or more), the carrier travels at a saturation speed V_(s) ≈10⁷ cm/sec according to equation (25):

    J.sub.C =q·n·V.sub.s                     (25)

where n is the number of carriers. The lower limit of the number n is also defined as a collector carrier density. (That is, n=N_(D) is the limit.) Because when the carrier density is less than the lower limit of n, neutrality of the charges in the semiconductor cannot be maintained by the carriers and a collector resistance is excessively increased (called a spatial charge effect). However, even if the spatial charge effect occurs, the positive holes are injected from the base (i.e., a base spread effect), and the spatial charge effect can be modeSated. When the J_(C) limit is given as 10,000 A/cm² the collector impurity concentration can be about 6×10¹⁵ cm⁻³ or more according to equation (25). However, when the J_(C) current limit is sacrificed by optical sensitivity of a photoelectric transducer element or the like, the lower limit is not restricted to the above-mentioned lower limit.

(Base Width)

Since a base width is determined by an h_(FE) requirement, the base width is not limited to a specific value.

(High-Speed BPT Operation)

Limits for high-speed BPT operations will be described below.

A cutoff frequency f_(T) of the BPT will be described below.

The cutoff frequency f_(T) is analytically represented by the following general equation:

    1/(2πf.sub.T)={(k·T)/(q·I.sub.C)+r.sub.EE }(C.sub.be +C.sub.bc)+π.sub.B +π.sub.C +r.sub.CC ·C.sub.bc(26)

where I_(C) is a collector current, C_(be) is an emitter-base capacitance, C_(bc) is a base-collector capacitance, r_(EE) is an emitter series resistance, r_(CC) is a collector series resistance, L_(B) is a travel time in the base, and L_(C) is a travel time in a collector depletion layer.

In order to increase the cutoff frequency f_(T) the terms on the right-hand side of equation (1) must be reduced.

The resistance r_(CC) is a resistance given by almost ohmic contact and depends on a surface concentration and an electrode. The impurity concentration of the surface is preferably n++.

The resistance r_(EE) is more influenced by the thickness of the n⁻ -type region than the thicknesses of the n⁺ -type regions 2 and 7 in FIG. 29.

The physical meaning of the cutoff frequency f_(T) is a time delay of the carrier injection from the emitter to the collector, and the time delay is represented by π_(ec), so that

    f.sub.T =1/(2ππτ.sub.ec)                         (27)

If f_(T=) 10 GHz, then τ_(ec=) 16 psec. A further detailed description will be made with reference to this time delay τ_(ec).

A travel time τ_(B) in the base is represented by the following equation:

    τ.sub.B =W.sub.B.sup.2 /(ηD.sub.B)                 (28)

When the base has a uniform doping distribution, η=2. However, when a potential gradient ε_(b1) is present, ηis given by the following equation:

    η≈2{1+(ε.sub.b1 /ε.sub.0).sup.3/2 }(29)

for ε₀ =2D_(B) /μ_(B) W_(B). Under the conditions that ε_(bi) =0, N_(B) =10¹⁸ cm⁻³, μ_(n) =300 cm² /sec V, and W_(E0) =0.1 μm, τ_(B) =13 psec. The travel time in the base is long because the carriers travel in the base by diffusion.

τVC is a travel speed in a collector depletion layer and can be represented as follows:

    TV.sub.C =W.sub.BC /2V.sub.s                               (30)

where WBC is a width of a baselcollector depletion layer, and V_(s) is a travel speed. Since the saturation speed of V_(s) is 107 cm/sec (if>10⁵ V/m), under the conditions of W_(BC) =0.1 μm, 0.5 μm, and 1 μm, the travel speeds τ_(VC) are respectively given as 0.5 psec, 2.5 psec, and 5 psec. The base and collector concentrations are determined so that W_(BC) is set to be 1 μm or less.

τ_(e) (emitter charge time) will be described below.

If C_(be) >>C_(bc) and an emitter area A_(E) =1.5×1.5 μm² are given, a high current up to J_(C) =10⁴ to 10⁵ A/cm² is used, so that I_(C) =J_(C) ·A_(E) =0.225 to 2.25 mA and kT/qI_(C) =111 to 11.1 Ωare obtained.

The emitter charge time can be obtained as: ##EQU18## Assuming that J_(C) =10⁴ A/cm² and kT/q·I_(C) ·γ>>γ_(EE), when τ_(e) <5 psec is given, then kT/q·I_(C) ·C_(be) <5 psec, and C_(be) <5×10¹² ·qIC/kT=4.5×10⁻¹⁴ F or less is derived. Therefore, for example, if C_(be) is 45 fF or less, τ_(e) is 5 psec or less. When C_(be) is 10 fF, τ_(e) is 1 psec. When C_(be) is 10 fF or less, the speed of τ_(e) does not pose any problem. τ_(e) does not pose any problem either in a high current.

The collector charge time τ_(c) is the last term of the right-hand side of equation (1) and is defined as:

    τ.sub.e =r.sub.CC (ε·S)/W.sub.BC      (32)

If S=4×4 μm², then L_(C) (1.66×10¹⁹)W_(BC) <1 psec, and then r_(CC) <6×10⁶ W_(BC). Even if W_(BC) =0.1 μm, no problem occurs when r_(CC) <60 Ωor less is given.

The base carrier concentration value need be larger than the number n₁ ² /N exp(V_(BE) +ΔE_(g) /kT) of injected minority carriers. In this case, neutrality of the charges can be assured, that is,

    N.sub.B>>n.sub.1.sup.2 /N.sub.B exp(V.sub.BE +ΔE.sub.g /kT)(33)

When the value N_(B) is about n_(i) exp(V+ΔE_(g) /2kT), the spatial charge effect appears, and the resistance in the base is abruptly increased.

A base-collector impurity density will be examined with reference to the width WBC of the base-collector deletion layer.

WBC and a width x_(p) of the depletion layer spreading in the base can be represented as follows:

    W.sub.BC = {(2ε.sub.s)/q}{(1/N.sub.B)+(1/N.sub.C)}×{(kT/q)·1.sub.n (N.sub.B ·N.sub.C /n.sub.i.sup.2)+V.sub.R }!.sup.1/2(34)

    x.sub.p ={N.sub.C /(N.sub.B +N.sub.C)}·W.sub.BC   (35)

FIG. 8 shows a relationship between the width x_(p) (μm) and the width W_(BC) of the depletion layer when the collector impurity densities N_(B) are 1×10¹⁶ cm⁻³, 3×10¹⁶ cm⁻³, and 6×20¹⁶ cm⁻³ under the conditions of ΔE_(g) =0.1 eV, room temperature, and V_(R) =5 V (constant). For example, if N_(C) =3×10¹⁶ cm⁻³ then W_(BC) =0.35, and π_(VC) in equation (29) is 1.75 psec, thus posing no problem. Since W_(B) must be 0.1 μm or less according to equation (27), when a voltage of 5 V is applied across the base-collector path, x_(p) satisfies at least condition x_(p) <W_(B). The concentration N_(B) must be almost 2×10¹⁷ cm⁻³ or more from FIG. 11.

As can be apparent from the above description, in the BPT of the present invention, when the current drive condition is given such that J_(C) =5×10⁴ A/cm², the impurity concentrations can be summarized as follows:

Collector impurity density: N_(C) >3×10¹⁶ cm⁻³

Base impurity density: N_(B) >2×10¹⁷ cm⁻³ (provided that the base width W_(B) =0.1 μm)

Emitter impurity density: N_(D) >2×17¹⁷ cm⁻³

When the emitter size, the base area, and the emitter-base voltage as a forward-biasing base-collector voltage are given as A_(E) =1.5×1.5 μm², A_(B) =4×4 μm², and +5 V, respectively,

C_(be) =3.6 fF

C_(bc) =4.9 fF

W_(BC) =0.35 μm

therefore, C_(VC) and τ_(c) can be sufficiently reduced. τ_(e) is basically in inverse proportion to τ_(c) and can be minimized in a large-current region. The lower limit of τ_(ec) is determined by the travel time τ_(B) of the base. Normally, τ_(B) determines the upper limit of f_(T) because motion is caused by diffusion.

(Embodiment 1)

As an embodiment of the present invention, a BPT having an arrangement as shown in FIG. 35 is formed by using an ion implantation method.

FIG. 36 is a graph showing impurity distributions and a Se distribution at a portion indicated by A-A' in FIG. 35. Referring to FIG. 12, the abscissa represents a depth, and the ordinate represents impurity concentrations of Ge, P, and B. Note that the ordinate represents logarithmic values while the abscissa represents linear indications.

Phosphorus (P) is an impurity in an emitter region, B is an impurity in a base region, and Ge represents the concentration of Si_(1-X) Ge_(X) in Si. In this case, P, B, and Ge does not always represent an absolute relationship. Note that in this embodiment, P and B are used as impurities. The impurities, however, are not limited to those used in this embodiment.

Since an Si density is about 5×10²² cm⁻³, Ge=5×10²¹ cm⁻³ if X=0.1.

For example, when ion implantation is performed under the conditions of SiO₂ =200 Å and Ge₇₄ of 150 keV to obtain a dose of Ge=5×10¹⁶ cm⁻², a Ge peak value of 5×10²¹ cm⁻³ is obtained at a portion separated from the surface of Si by 700 Å.

A relationship between the Ge peak value and the depth is uniquely determined by a relationship between the dose and the acceleration voltage, The thickness of a transition region between Si and Si_(1-X) Ge_(X) is mainly determined by the acceleration voltage and annealing in the following process.

An emitter-base junction surface (E-B surface) is preferably close to a Ge peak concentration.

A ratio γ of minority carriers to the base with respect to-those to the emitter at the E-B junction is approximated by the following equation:

    γ=(L.sub.p D.sub.n N.sub.D /W.sub.B D.sub.P N.sub.A)exp(ΔE.sub.g /kT)                           (1)

where N_(D) /N_(A) is a doping ratio between the emitter and the base, D_(N) /D_(P) is a diffusion coefficient ratio between the base and emitter minority carriers, W_(B) is a base width, and ΔE_(g) is an emitter-base band gap reduction width. Exp(ΔE_(g) /kT) represents an effect of the E-B heterojunction. Assuming that ΔE_(g) ≈0.1 V and a temperature is room temperature, exp(ΔE_(g) /kT) ≈54.5, and ΔE_(C) ≈0 in Si_(1-X) Ge_(X). Therefore, ΔE_(g) ≈ΔE_(V) is obtained.

In a normal BPT constituted. by only Si, a base current is determined by a recombination current J_(Brec) of electrons from an emitter and an injection current J_(Binj) from a base to the emitter. Since J_(B) =J_(Brec) +J_(Binj), h_(FE) ≈J_(C) /J_(Brec) is obtained when J_(Brec) >>J_(Binj). That is, h_(FE) is increased as compared with that of a homojunction BPT. In a conventional Si_(1-X) Ge_(X) heterojunction BPT, however, a stress is increased, and dislocation occurs in an extreme case. In addition, since many recombination centers are present in the interface between Si_(1-X) Ge_(X) and Si, an overcurrent flows. I_(B) is increased at a low current side, and h_(FE) is significantly reduced in a region in which I_(C) is small. FIG. 37 shows an improvement according to the present invention. Referring to FIG. 37, A represents a conventional HBT; and B, an HBT of the present invention.

In this embodiment, X is preferably 0.5 or less. If X exceeds 0.5, it is difficult to manufacture a good BPT. In addition, X of 0.5 or less is sufficient in the BPT.

A process of manufacturing the BPT of this embodiment will be described below.

(1) A buried region (n⁺) 2 is formed on a p- or n-type substrate 1.

(2) An n⁻ -type region is formed by an epitaxial technique or the like.

(3) A 200-Å thick oxide film and a 1,500-Å thick nitride film are deposited on the entire surface.

(4) A recessed portion of an element isolation region 101 is formed by patterning and etching the nitride film.

(5) A channel stopper 6 is formed by ion implantation.

(6) The element isolation region 101 is formed by selectively oxidizing only the recessed portion.

(7) The nitride film and SiO₂ (200 Å) are removed.

(8) An oxide film 102 is formed on the entire surface.

(9) Ge is ion-implanted (Ge concentration: 5×10¹⁶ /150 keV) on the entire surface. Ge penetrates in only a thin portion of the oxide film.

(10) Annealing is performed at 1,100° C. for one hour (Ge depth X_(j) =0.2 μm).

(11) B is ion-implanted (B concentration: 1×10¹⁴ /5 keV) on the entire surface. B penetrates in only a thin portion of the oxide film.

(12) Annealing is performed at 850° C. for 30 minutes (B depth X_(j) =0.1 μm).

(13) The oxide film is patterned and etched to form an opening in a portion of an emitter 5.

(14) Polysilicon is deposited on the entire surface to form the emitter 5.

(15) P is ion-implanted (concentration: 5×10¹⁵ /60 keV).

(16) Annealing is performed.

(17) Polysilicon is processed.

(18) Additional annealing is performed.

(19) SiO₂ 103 is formed.

(20) An opening of a contact portion of the SiO₂ 103 is formed.

(21) Al--Si is deposited in the opening of the contact portion.

(22) The deposited Al--Si is patterned.

(23) Al--Si is alloyed.

The manufacturing process of this embodiment is as briefly described above. The most important part of this embodiment is a base region formation step described in item (5).

Although a heterojunction having an inclined transition region can be formed without using ion implantation, ion implantation is preferably used.

For example, a heterojunction having an inclined transition region can be formed by a vapor-phase growth method. In this case, an H₂ +SiH₂ Cl₂ +GeH₄ -based gas, for example, is used, and a gas concentration of GeH₄ is continuously changed to form a base region. If a temperature is 900° C. or less, a gas pressure is 100 Torr or less, and an epitaxial rate is 500 Å/min, the heterojunction can be formed more easily. In addition, the heterojunction is assumed to be obtained by an epitaxial technique in vacuum.

Ion implantation, however, is superior because a mixed crystal ratio can be stably set, a Ge concentration distribution can be correctly reproduced, and a mass production property is good.

In addition, since Si is in the form of amorphous after ion implantation of Ge, it can be easily subjected to solid-phase epitaxy, thereby reducing the temperature of a process. Furthermore, since a diffusion constant of Ge or Si is very small, the Ge concentration distribution does not almost change upon ion implantation in an annealing step at 1,000° C. or less.

As described above, formation of a mixed crystal of Si_(1-X) Ge_(X) by ion implantation has may advantages.

A distribution function with respect to a depth direction X in a semiconductor upon ion implantation is approximated as follows:

    N(X)≈(N.sub.o /2.5R.sub.p)exp(-(X-R.sub.p).sup.2 /(ΔR.sub.p).sup.2)                                  (2)

where N_(o) is an implantation amount per unit area, R_(p) is a peak position, and ΔR_(p) is a distance from the peak to one diffusion amount (1σ). R_(p) andΔR_(p) are changed by an acceleration voltage.

By forming the hetero BPT as described above, a point defect and dislocation in the hetero interface can be prevented to increase the current gain.

(Embodiment 2)

FIG. 38 shows another embodiment of the present invention.

A hetero BPT according to this embodiment is the same as that of Embodiment 1 except that an emitter has a buried structure, a substrate 1 has an n conductivity type, and no buried region is formed.

In such a hetero BPT, h_(FE) can be increased as in the hetero BPT shown in Embodiment 1.

(Embodiment 3)

FIG. 39 shows the third embodiment of the present invention.

A hetero BPT according to this embodiment differs from Embodiment 1 in that an n⁺ -type contact 8 is formed. The n⁺ -type contact 8 is formed to reduce an ohmic resistance of an emitter 5. An impurity concentration of the n⁺ -type contact 8 is preferably 10¹⁹ to 10²¹ cm⁻³.

In such a hetero BPT, h_(FE) can be increased as in the hetero BPT shown in Embodiment 1.

(Embodiment 4)

FIG. 40 shows an embodiment in which an emitter 5 is formed on a substrate.

In such a hetero BPT, h_(FE) can be increased.

(Embodiment 5)

FIG. 41 is a circuit diagram showing a case in which the hetero BPT shown in the first embodiment is applied to a solid-state image pickup apparatus in patent application (2) filed by the present applicant on Dec. 21, 1987. Referring to FIG. 41, the hetero BPT shown in Embodiment 1 is used in a portion indicated by Tr.

That is, in this embodiment, the hetero BPT is used as a photoelectric transducer element.

Since the present invention can improve f_(T) (frequency) of a semiconductor device, the present invention is very effective when it is applied to a photoelectric transducer apparatus.

In the photoelectric transducer apparatus, F_(T) is determined by a read rate.

A current photoelectric transducer apparatus (area sensor) has 500×640 elements. An HD (High Division: an area sensor corresponding to a high vision) has 1,000×2,000 elements. In an operation of a current television, a horizontal scanning period HT≈50 μsec, and a horizontal blanking period HALK is 8 to 10 μsec. In the HD, however, HT=3 to 3.7 μsec and HBLK≈26 μsec. Horizontal scanning is conventionally T_(H) =50 μsec/640=80 nsec, while that of the HD is T_(H) =26 nsec/2,000=13 nsec.

A frequency must be at least six times that of a current television. That is, since a frequency is currently f_(T) ≈1 to 2 GHz, f_(T) >6 to 16 GHz.

For the above reasons, ΔE_(g) is preferably 0.15 eV or less.

When the area sensor shown in FIG. 41 is to be used as a color camera, optical information of the same photoelectric transducer element is read a plurality of times. Since information is read from the same element a plurality of times, a ratio between electrical outputs upon first reading and reading from the second time is important. If this value is reduced, correction must be performed.

The ratio between the first and second read outputs is defined as a nondestructive degree.

The nondestructive degree is given by the following equation:

    Nondestructive degree=(C.sub.tot ×h.sub.FE )/(C.sub.tot ×h.sub.FE +C.sub.V)

where C_(tot) is a total capacitance connected to the bases of the photoelectric transducer elements indicated by Tr in FIG. 7 and is determined by a base-emitter capacitance C_(be), a base-collector capacitance C_(bc), and C_(ox). C_(V) is a stray capacitance of read lines represented by VL₁ . . . VL_(n). Note that C is not present depending on circuit systems.

The nondestructive degree can be easily improved by increasing h_(FE) . That is, when h_(FE) is increased, the nondestructive degree can be increased.

In the HD-corresponding area sensor, C_(tot) =10 fF and C_(V) =2.5 PF. Therefore, in order to obtain the nondestructive degree of 0.90 or more, for example, h_(FE) must be 2,250 or more. In order to assure a sufficient nondestructive degree, h_(FE) must be 2,000 or more.

To the contrary, in a conventional homojunction BPT, for example, h_(FE) is about 1,000. Therefore, no sufficient nondestructive degree can be obtained. In the semiconductor device of the present invention, however, a high nondestructive degree is obtained since h_(FE) can be sufficiently increased.

The nondestructive degree is more preferably 0.98 or more. At this time, h_(FE) must be about 10,000. This value cannot be achieved by a conventional hetero BPT or homo BPT. That is, this value can be achieved by only the hetero BPT of the present invention.

Note that although the area sensor is exemplified in this embodiment, the present invention can be applied to a line sensor.

As has been described above, according to the present invention, since a heterojunction having an inclined transition region is formed in an interface between an Si_(1-X) Ge_(X) mixed crystal and Si of a semiconductor device, the number of point defects and dislocations can be significantly reduced. Therefore, a current gain can be significantly. improved in a region having a very small collector current.

In addition, according to the present invention, an ion implantation method is used. Therefore, a semiconductor device can be very easily manufactured and mass production can be facilitated.

That is, according to the Embodiments 1-5, a high-performance semiconductor device and a photoelectric transducer apparatus using the same can be provided at very low cost.

(Embodiment 6)

A BPT having the same structure as in FIG. 42 was manufactured as an embodiment of the present invention by using an ion doping method.

The manufacturing process of the BPT of this embodiment is shown.

1 An n⁺ -type buried region 2 was formed in a p-type substrate 1.

2 An n⁻ -type region 3 was formed on the buried region 2 by epitaxial techniques.

3 An n⁺ -type region 7 was formed to decrease a collector resistance of the BPT.

4 An element isolation region 101 was formed together with a channel stopper region 6.

5 A p-type impurity region containing Ge and B or Ga was formed on the n -type region 3 (formation of the base region) by ion implantation. When the distribution of the doped impurity in the n⁻ -type region 3 was measured, results shown in FIG. 2 were obtained, and this will be described in detail later.

6 After an emitter contact hole is formed, impurity-doped potysilicon or single-crystal silicon was deposited by low-pressure chemical vapor deposition (LPCVD) or epitaxy, and the region 5 was patterned (formation of the emitter region). In this case, when P was doped by ion implantation. When an impurity (P) concentration was measured, the distribution shown in FIG. 43 was obtained.

7 An insulator 103 was deposited, contact holes were formed therein, a wiring electrode material such as Al was deposited, and the electrode material was patterned.

8 An insulator 104 was deposited, and external lead openings are formed.

The above general steps constitute the manufacturing process of this embodiment. The most important step in this embodiment is step 5, i.e., the base formation step. The ion implantation method has advantages in that a mixed crystal can be stably set, the Ge concentration distribution can be accurately reproduced, and mass production can be facilitated. In addition, after Ge is ion-implanted, Si is amorphous, and solid-phase epitaxial growth can be easily performed. Therefore, a low-temperature process can be performed. In addition, Ge has a small diffusion constant in Si and an advantage in that the Ge concentration distribution at the time of ion implantation is rarely changed when annealing is performed at 1,000° C. or less.

A distribution function of tie semiconductor in a direction X of depth during ion implantation can be approximated as follows:

    N(X)≈(N.sub.o /2.5R.sub.p) exp(-(X-R.sub.p)2/(ΔR.sub.p).sup.2)(18)

where N_(o) is an implantation amount per unit area, R_(p) is a peak position, and ARp is a distance at which an implantation amount is reduced to one diffusion amount (1 σ) from the peak. The position Rp and the distance ΔR_(p) are changed in accordance with an acceleration voltage.

FIG. 43 is a graph showing B, Ge, and P distributions when ion implantation is performed as described above. The depth (relative value) is plotted along the abscissa, and the atomic number/cm⁻³ is plotted along the ordinate in FIG. 43.

As a result of formation of the hetero BPT described above, there is provided a semiconductor device having a high base-emitter breakdown voltage, a small base-emitter capacitance, and a high h_(FE) .

(Embodiment 7)

FIG. 44 is a view showing another embodiment of the present invention.

A hetero BPT according-to this embodiment is different from that of Embodiment 6 except that an emitter has a dried structure, a substrate 1 has an n conductivity type, and a buried region is eliminated.

The same effect as in the BPT of Embodiment 1 can be obtained in this hereto BPT.

(Embodiment 8)

FIG. 45 shows a structure wherein an emitter region 5 is formed on a substrate 1 side.

The same effect as in the BPT of Embodiment 6 can be obtained in this hereto BPT.

(Embodiment 9)

FIG. 46 is a view showing the 8th embodiment of the present invention.

A hetero BPT of this embodiment is different from that of Embodiment 6 except that an n⁺ -type contact 8 is formed. The n⁺ -type contact 8 is formed to decrease an ohmic resistance of an emitter 5. The n⁺ -type contact 8 has an impurity concentration of 10²⁰ cm⁻³.

The same effect as in the BPT of Embodiment 6 can be obtained in this hetero BPT.

(Embodiment 10)

FIG. 41 is a circuit diagram wherein the BPT shown in Embodiment 6 is used in a solid-state image pickup apparatus. The BPT shown in Embodiment 6 is used in a portion represented by Tr in FIG. 41.

In this embodiment, the BPT shown in Embodiment 6 is used as a photoelectric transducer element.

According to the present invention, f_(T) (frequency) of the semiconductor device can be increased. Therefore, when the semiconductor device is used in the photoelectric transducer apparatus, it is very effective.

The frequency f_(T) in the photoelectric transducer apparatus is determined by a read rate.

An existing photoelectric transducer apparatus (area sensor) has 500×640 elements. An HD (High Diffusion; an area sensor corresponding to high vision) has 1,000×2,000 elements. In an existing television operation scheme, a horizontal scan period is HT≈50 μsec, and a horizontal blanking period HALK is 8 to 10 μsec. In the HD, HT=3 to 3.7 μsec, and HBLK=26 μsec. When horizontal scanning is taken into consideration, T_(H) =50 μsec/640=80 nsec in the conventional arrangement, whereas T_(H) =26 nsec/2,000=13 nsec in the HD.

The frequency is required to be at least six times. That is, in the existing scheme, f_(T)≈ 1 to 2 GHz. Therefore, f_(T) >6 to 16 GHz or more is required.

When an area sensor shown in FIG. 41 is used as a color camera, optical information of the same photoelectric transducer element is read a plurality of times. In this case, since the optical information is read from the same element by a plurality of times, a ratio of the first read output to the second read output is very important. When this ratio is reduced, correction is required.

The ratio of the first read output to the second read output is defined as a nondestructive degree.

The nondestructive degree ismrepresented by the following equation.

    Nondestructive Degree=(C.sub.tot ×h.sub.FE )/(C.sub.tot ×h.sub.FE +C.sub.V)

where C_(tot) is a total capacitance connected to the bases of the photoelectric transducer elements represented by Tr in FIG. 41. The total capacitance C_(tot) is determined by a represented by VL₁ . . . VL_(n). Note that Cox is not present C_(bc), and C_(0x). C_(V) is a stray capacitance of read lines base-emitter capacitance Cbe, a base-collector capacitance depending on circuit systems.

The nondestructive degree can be easily improved by increasing the current gain h_(FE) . That is, when h_(FE) is increased, the nondestructive degree can be increased.

C_(tot) is represented by the capacitances of the BPT SENSOR, i.e., the base-emitter capacitance and the base-collector capacitance. That is, C_(tot) =C_(be) +C_(bc). Normally, if C_(be) >C_(bc) and the impurity concentration of the base is increased, C_(be) becomes the main factor for determining the nondestructive degree.

In the above equation, C_(V) is a stray capacitance of vertical lines, and h_(FE) is a BPT current gain. Since condition C_(tot) <10 fF is given for two million pixels, C_(be) <10 fF must be satisfied. Whether C_(be) <10 fF is established is determined by an emitter concentration. If the emitter size is given as 1.5 (an area S),

C_(be) =ε·S/d (where d is the thickness of the depletion layer)

    ε=ε.sub.s ·ε.sub.) (ε.sub.s =11.8 in Si)

Since S=2.25×10⁻⁸ cm² is given, d ≧235 Å. FIG. 47 shows upper limit values of concentrations N_(D) (impurity concentrations of the emitter region) when the impurity concentrations of the emitter region with respect to the impurity concentration of the base region are given as C_(be) =10 fF and 5 fF. For example, if N_(B) =10¹⁹ cm⁻³, N_(D) =3×10¹⁸ cm⁻³ or less for C_(be) =10 fF, and 6.3×10¹⁷ cm⁻³ for C_(be) =5 fF.

In the area sensor corresponding to the HD, since C_(tot) =10 fF and C_(V) =2.5 pF, h_(FE) must be 2,250 or more to obtain the nondestructive degree of 0.90 or more. In order to assure a sufficiently high nondestructive degree, h_(FE) is assumed to be 2,000 or more.

To the contrary, since h_(FE) is about 1,000 in a conventional homo BPT, a sufficiently high nondestructive degree could not be obtained. In the semiconductor device of the present invention, since a sufficient high h_(FE) can be obtained, an excellent nondestructive degree can be obtained.

More preferably, the nondestructive degree is 0.98 or more. In this case, h_(FE) must be about 10,000. In the conventional homo BPT, this value cannot be obtained but can be obtained by only the BPT of the present invention.

The above embodiment exemplifies an area sensor, but the present invention is also applicable to a line sensor.

As has been described above, according to the present embodiments, the impurity concentration of the emitter can be suppressed to be 10¹⁹ cm⁻³ or less. Therefore, band-gap narrowing can be suppressed, and efficiency of injection from the emitter can be improved.

A band-gap narrowing effect obtained by a high boron concentration of the base and a mixed crystal effect obtained by using B together with Ge can cause to decrease a base band gap smaller than the emitter band gap. Therefore, the h_(FE) of the BPT can be increased. In addition, the lattice distortion can be moderated, and therefore, defects at the interface can be minimized.

(Embodiment 11)

FIG. 48 is a view showing a semiconductor device according to an embodiment of the present invention. The same reference numerals as in FIG. 4 denote the same parts in FIG. 48. The semiconductor device of this embodiment is different from the semiconductor device of FIG. 4 in that an emitter region 5 is formed by growing a single crystal by epitaxial techniques or the like and the oxide 102 is not formed between a base region 4 and the emitter region 5.

A manufacturing process of the BPT of this embodiment is shown below.

1 An n⁺ -type buried region 2 (impurity concentration: 1×10¹⁶ to 10¹⁹ cm⁻³) is formed on a p- or n-type substrate 1 by ion implantation, impurity diffusion, or the like.

2 An⁻ -type region 3 (impurity concentration: 1×10¹⁴ to 10¹⁷ cm⁻³) is formed by epitaxial techniques.

3 A recessed portion of an element isolation region 101 is formed.

4 A channel stopper 6 is formed by ion implantation.

5 An element isolation region 101 is formed by selectively oxidizing the recessed portion.

6 A base region 4 (impurity concentration: 1×10¹⁶ to 10¹⁹ cm⁻³) is formed.

7 An oxide film 102 is formed on the entire surface.

8 A hole serving as an emitter contact hole is formed in the oxide film 102 by etching.

9 An n⁺ -type region (impurity concentration: 1×10¹⁷ to 10²¹ cm⁻³) serving as an emitter is formed by epitaxial techniques (self-aligned emitter region).

10 SiO₂ 103 is formed.

11 Electrodes 200 are formed.

12 Passivation 104 is formed.

The above steps constitute the manufacturing process of this embodiment. The most important steps in this embodiment are steps 8 and 9 which represent the manufacturing process of the emitter region. In these steps, after the silicon oxide film on the silicon surface constituting the surface base region is removed, single-crystal silicon is formed as the emitter region on the surface base region.

Single-crystal silicon can be grown by an epitaxial method using SiH₄, Si₂ H₆, SICl₄, SiHCl₃, or SiH₃ Cl gas and a carrier gas. In this epitaxial method, the Si surface must be cleaned beforehand by a gas mixture of H₂ gas or HCl gas, or a gas mixture of a halogen gas such as HBr or HF. Cleaning and crystal growth can be performed at a low temperature and a low pressure of 10 to 100 Torr. The lower the temperature of cleaning and crystal growth is, the higher the packing density is. Surface cleaning can be performed by sputtering of a gas (e.g., Ar) or heating. In epitaxial growth, it is very important to perform perfect surface cleaning.

In order to cause a single crystal to epitaxially grow, the oxide film on the base region 4 must be removed. Although it is difficult to perfectly remove the oxide film, more perfect epitaxial growth can be performed in a smaller content of oxygen contained in the base surface.

The current gain h_(FE) of the resultant BPT was measured, and a higher gain than the conventional BPT could be obtained.

Measurement results of V_(BE) dependency of h_(FE) are shown in FIG. 49. As can be apparent from FIG. 49, the current gain h_(FE) rarely depended on V_(BE) under the condition exp(V_(BE) /kT)>>1.

(Embodiment 12)

FIG. 50 shows another embodiment of the present invention.

A BPT of this embodiment is different from Embodiment 11 except that an emitter has a buried structure.

Since an end portion of an emitter region 5 in the BPT of Embodiment 11 is deposited on the oxide film (insulator 102), the substance on the oxide film is polysilicon. However in Embodiment 2, the entire emitter region 5 consists of a single crystal, and the current gain h_(FE) can be further increased. (Embodiment 13)

FIG. 51 is a view showing the embodiment 13 of the present invention. This embodiment exemplifies a BPT in which an element isolation region is a groove isolation structure.

The current gain h_(FE) of this BPT can also be increased.

(Embodiment 14)

FIG. 41 is a circuit diagram wherein the BPT of the embodiment 11 is used in a solid-state image pickup apparatus. Referring to FIG. 41, the BPT shown in Embodiment 11 is used in a portion represented by Tr.

In this embodiment, the BPT is used as a photoelectric transducer element.

For example, when an area sensor shown in FIG. 6 is used as a color camera, optical information of the same photoelectric transducer element is read a plurality of times. In this case, since the optical information is read from the same element by a plurality of times, a ratio of the first read output to the second read output is very important. When this ratio is reduced, correction is required.

If the ratio of the first read output to the second read output is defined as a nondestructive degree, the nondestructive degree is represented by the following equation:

    Nondestructive Degree=(C.sub.tot ×h.sub.FE )/(C.sub.tot ×h.sub.FE+C.sub.v)

where C_(tot) is a total capacitahce connected to the bases of the photoelectric transducer elements represented by Tr in FIG. 4. The total capacitance C_(tot) is determined by a base-emitter capacitance C_(be), a base-collector capacitance C_(bc), and C_(ox). C_(V) is a stray capacitance of read lines represented by VL₁ . . . VL_(n). Note that C_(ox) is not present depending on circuit systems.

The nondestructive degree can be easily improved by increasing the current gain h_(FE) . That is, when h_(FE) is increased, the nondestructive degree can be increased.

This embodiment exemplifies an area sensor. However, the present invention is also applicable to a line sensor.

As has been described above, according to the present Embodiments 11-14, since an oxide film is not formed between emitter and base regions, characteristic variations between the BPTs can be eliminated. Therefore, when the BPT is used as a line sensor, an area sensor, or any other semiconductor device, the product yield can be increased.

In addition, according to the present invention, since the emitter region consists of single-crystal silicon, the current gain h_(FE) can be higher than that of the conventional structure although the oxide film is not formed between the emitter and base regions.

According to the present invention, there are provided a high-performance semiconductor device and a photoelectric transducer apparatus using the same at low cost.

(Embodiment 15)

As an embodiment of the present invention, a BPT having an arrangement as shown in FIG. 35 is formed by using an ion implantation method.

FIG. 24 is a graph showing impurity distributions and a Ge distribution at a portion indicated by A-A' in FIG. 35. Referring to FIG. 24, the abscissa .represents a depth, and the ordinate represents impurity concentrations of Ge, P, and B. Note that the ordinate represents logarithmic values while the abscissa represents linear indications. Note that arsenic (As) is an impurity of an emitter region, B is an impurity of a base region, and Ge represents the concentration of Si_(1-X) Ge_(X) in Si. In this case, As, B, and Ge does not always represent an absolute relationship.

Note that in this embodiment, As and B are used as impurities. The impurities, however, are not limited to those used in this embodiment.

Since an Si density is about 5×10²² cm⁻³, Ge=5×10²¹ cm⁻³ if X=0.1.

For example, when ion implantation is performed under the conditions of SiO₂ =200 Å and Ge₇₄ of 150 keV to obtain a dose of Ge=5×10¹⁶ cm⁻², a Ge peak value of 5×10²¹ cm ⁻³ is obtained at a portion separated from the surface of Si by 700 Å.

A relationship between the Ge peak value and the depth is uniquely determined by a relationship between the dose and the acceleration voltage. The thickness of a transition region between Si and Si_(1-X) Ge_(X) is mainly determined by the acceleration voltage and annealing in the following process.

An emitter-base junction surface (E-B surface) is preferably close to a Ge peak concentration.

A ratio γ of minority carriers to the base with respect to those to the emitter at the E-B junction is approximated by the following equation:

    γ=(L.sub.p D.sub.n N.sub.D /W.sub.B D.sub.P N.sub.A)exp(ΔE.sub.g /kT)                           (26)

where

N_(D/N) _(A) ; a doping ratio between the emitter and the base,

D_(N/D) _(P) ; a diffusion coefficient ratio between the base and emitter minority carriers,

W_(B) ; a base width, and

E_(g) ; an emitter-base band gap reduction width.

In addition,

exp(ΔE_(g) /kT) is an effect of an E-B heterojunction.

Assuming that ΔE_(g) ≈0.1 V and a temperature is room temperature, exp(ΔE_(g) /kT) ≈54.5, and ΔE_(C) ≈0 in Si_(1-X) Ge_(X), Therefore, ΔE_(g) ≈ΔE_(V) is obtained.

In a normal BPT constituted by only Si, a base current is determined by a recombination current J_(Brec) of electrons from an emitter and an injection current JBinj from a base to the emitter.

Since

J_(B) =J_(Brec) +J_(Binj),

h_(FE) ≈J_(C) /J_(Brec) is obtained when J_(Brec) >>J_(Binj). That is, h_(FE) is increased as compared with that of a homojunction BPT. In a conventional Si_(1-X) Ge_(X) heterojunction BPT, however, a stress is increased, and dislocation occurs in an extreme case. In addition, since many recombination centers are present in the interface between Si_(1-X) Ge_(X) and Si, an overcurrent flows. I_(B) is increased at a low current side, and h_(FE) is significantly reduced in a region in which I_(C) is small. FIG. 3 shows an improvement according to the present invention. Referring to FIG. 3, A represents a conventional HBT; and B, an HBT of the present invention.

In this embodiment, X is preferably 0.5 or less. If X exceeds 0.5, it is difficult to manufacture a good BPT. In addition, X of 0.5 or less is sufficient in the BPT.

A process of manufacturing the BPT of this embodiment will be described below.

1 A buried region (n⁺) 2 is formed on a p- or n-type substrate 1.

2 An n⁻ -type region is formed by an epitaxial technique or the like.

3 An n+-type region 7 for reducing a collector resistance of a BPT is formed.

4 An element isolation region 101 is formed together with a channel stopper region.

5 A p-type impurity such as Ge and B or Ga or the like is formed on an n⁻ -type region 3 by ion implantation to serve as a base region.

6 After a contact as an emitter is formed, polycrystalline or single-crystal silicon is deposited by low-pressure chemical vapor deposition or an epitaxial device, an impurity is doped, and a region 5 is patterned to form an emitter region.

7 After an insulator 103 is deposited, a contact hole is formed, and an Al material is deposited and patterned.

8 After an insulator 104 is deposited, an external extraction hole is formed. The manufacturing process of this embodiment is as briefly described above. The most important part of this embodiment is a base region formation step described in item 5.

Although a heterojunction having an inclined transition region can be formed without using ion implantation, ion implantation is preferably used.

For example, a heterojunction having an inclined transition region can be formed by a vapor-phase growth method. In this case, an H₂ +SiH₂ Cl₂ +GeH₄ -based gas, for example, is used, and a gas concentration of GeH₄ is continuously changed to form a base region. If a temperature is 900° C. or less, a gas pressure is 100 Torr or less, and an epitaxial rate is 500 Å/min, the heterojunction can be formed more easily. In addition, the heterojunction is assumed to be obtained by an epitaxial technique in vacuum.

Ion implantation, however, is superior because a mixed crystal ratio can be stably set, a Ge concentration distribution can be correctly reproduced, and a mass production property is good.

In addition, since Si is in the amorphous form after ion implantation of Ge, it can be easily subjected to solid-phase. epitaxy, thereby reducing the temperature of a process. Furthermore, since a diffusion constant of Ge or Si is very small, the Ge concentration distribution does not almost change upon ion implantation in an annealing step at 1,000° C. or less.

As described above, formation of a mixed crystal of Si_(1-X) Ge_(X) by ion implantation has may advantages.

A distribution function with respect to a depth direction X in a semiconductor upon ion implantation is approximated as follows:

    N(X)≈(N.sub.o /2.5R.sub.P)exp(-(X-R.sub.P).sup.2 /(ΔR.sub.P).sup.2)                                  (27)

where N_(o) is an implantation amount per unit area, R_(P) is a peak position, and ΔR_(P) is a distance from the peak to one diffusion amount (1 .o slashed.). R_(P) and ΔR_(P) are changed by an acceleration voltage.

By forming the hetero BPT as described above, a point defect and dislocation in the hetero interface can be prevented to increase the current gain.

(Embodiment 16)

Another embodiment will be described below.

In the present invention, since Ge is ion-implanted in Si, an arbitrary impurity distribution can be easily formed by changing an acceleration voltage of ions. Therefore, by changing an acceleration voltage upon Ge ion implantation, X of Si_(1-X) Ge_(X) can take different values in shallow and deep regions of the base.

In this embodiment, by changing an acceleration voltage upon Ge ion implantation, a transistor in which X of Si_(1-X) Ge_(X) is larger in a deep region than in a shallow region of the base is manufactured.

FIG. 52 is a potential view of a BPT according to this embodiment. In the base close to the emitter, a band gap is E_(g) '(ΔE_(g) =E_(g) -E_(g) ") which is smaller than that of the emitter. In the base close to the collector, the band gap is E_(g) "(E_(g) '>E_(g) ").

A propagation velocity of minority carriers in the base upon normal diffusion can be approximated by the following relation:

    V.sub.di ≈D.sub.n /W.sub.E =(kT/q)·(1/W.sub.B)·μ.sub.n          (28)

As shown in FIG. 52, when a potential inclination is present in the base, the minority carriers are moved by a drift, and the propagation velocity can be approximated by the following relation:

    V.sub.di ≈μ.sub.n E=μ.sub.n ·ΔE.sub.g2 /W.sub.B(29)

Therefore, the drift effect is significant within the. range of ΔEg₂ >kT/q. Since kT/q is 0.025 eV at room temperature, an effect is obtained if the value is larger than that (i.e., a transit velocity V is increased). This effect becomes significant at a lower temperature. When the transit velocity V is increased, carriers in the base move rapidly to improve RF characteristics. In addition, in a drift structure, a moving velocity is not influenced by an operation temperature of a BPT.

(Embodiment 17)

FIG. 53 shows the embodiment 17 of the present invention.

A hetero BPT according to this embodiment is the same as that of Embodiment 1 except that an emitter has a buried structure, a substrate 1 has an n conductivity type, and no buried region is formed.

In such a hetero BPT, h_(FE) can be increased as in the hetero BPT shown in Embodiment 16.

(Embodiment 18)

FIG. 54 shows an embodiment in which an emitter 5 is formed on a substrate.

In such a hetero BPT, h_(FE) can be increased as in the hetero BPT of Embodiment 16.

(Embodiment 19)

FIG. 41 is a circuit diagram showing a case in which the hetero BPT shown in the first embodiment is applied to a solid-state image pickup apparatus. Referring to FIG. 41, the hetero BPT shown in Embodiment 16 is used in a portion indicated by Tr.

That is, in this embodiment, the hetero BPT is used as a photoelectric transducer element.

Since the present invention can improve f_(T) (frequency) 10 of a semiconductor device, the present invention is very effective when it is applied to a photoelectric transducer apparatus.

In the photoelectric transducer apparatus, f_(T) is determined by a read rate.

A current photoelectric transducer apparatus (area sensor) has 500×640 elements. An HD (High Division: an area sensor corresponding to a high vision) has 1,000×2,000 elements. In an operation of a current television, a horizontal scanning period HT≈50 μsec, and a horizontal blanking period HALK is 8 to 10 μsec. In the HD, however, HT=3 to 3.7 μsec and HBLK=26 μsec. Horizontal scanning is conventionally T_(H) =50 μsec/640=80 μsec, while that of the HD is T_(H) =26 nsec/2,000 =13 nsec.

A frequency must be at least six times that of a current television. That is, since a frequency is currently f_(T) ≈1 to 2 GHz, f_(T) >6 to 16 GHz.

For the above reasons, ΔE_(g) is preferably 0.15 eV or less.

When the area sensor shown in FIG. 41 is to be used as a color camera, optical information of the same photoelectric transducer element is read a plurality of times. Since information is read from the same element a plurality of times, a ratio between electrical outputs upon first reading and reading from the second time is important. If this value is reduced, correction must be performed.

The ratio between the first and second read outputs is defined as a nondestructive degree.

The nondestructive degree is given by the following equation:

Nondestructive degree=(C_(tot) ×h_(FE) )/(C_(tot) ×h_(FE) +C_(V)) where C_(tot) is a total capacitance connected to the bases of the photoelectric transducer elements indicated by Tr in FIG. 41 and is determined by a base-emitter capacitance Cbe, a base-collector capacitance C_(bc), and C_(ox). C_(V) is a stray capacitance of read lines represented by VL₁. . . VL_(n). Note that C_(ox) is not present depending on circuit systems.

The nondestructive degree can be easily improved by increasing h_(FE) . That is, when h_(FE) is increased, the nondestructive degree can be increased.

In the HD-corresponding area sensor, C_(tot) =10 fF and C_(V) =2.5 PF. Therefore, in order to obtain the nondestructive degree of 0.90 or more, for example, h_(FE) must be 2,250 or more. In order to assure a sufficient nondestructive degree, h_(FE) must be 2,000 or more.

To the contrary, in a conventional homojunction BPT, for example, h_(FE) is about 1,000. Therefore, no sufficient nondestructive degree can be obtained. In the semiconductor device of the present invention, however, a high nondestructive degree is obtained since h_(FE) can be sufficiently increased.

The nondestructive degree is more preferably 0.98 or 10 more. At this time, h_(FE) must be about 10,000. This value cannot be achieved by a conventional hetero BPT or homo BPT. That is, this value can be achieved by only the hetero BPT of the present invention.

Note that although the area sensor is exemplified in this embodiment, the present invention can be applied to a line sensor.

As has been described above, according to the present Embodiments 16-18, since a heterojunction having an inclined transition region is formed in an interface between an Si_(1-X) Ge_(X) mixed crystal and Si of a semiconductor device, the number of point defects and dislocations can be significantly reduced. Therefore, a current gain can be significantly improved in a region having a very small collector current.

In addition, according to the present invention, an ion implantation method is used. Therefore, a semiconductor device can be very easily manufactured to maintain a good mass production property.

That is, according to the present invention, a high-performance semiconductor device and a photoelectric transducer apparatus using the same can be provided at very low cost.

(Embodiment 19)

FIG. 55 is a schematic sectional view showing a semiconductor device according to an embodiment of the present invention. The same reference numerals as in FIG. 6 denote the same parts in FIG. 55. The semiconductor device of this embodiment differs from the convention semiconductor device in the following points.

a The silicon oxide film formed at the interface between a base region 4 and an emitter region 5 is eliminated.

b The emitter region 5 is made of single-crystal silicon.

c An impurity concentration near an interface between the emitter region and the base region 4 in the emitter region 5 is set to be 1×10¹⁹ cm⁻³ or less to prevent band-gap narrowing.

d An emitter region 9 having a high impurity concentration of 1×10¹⁹ to 1×10²¹ cm⁻³ is formed between an emitter electrode 200 and the emitter region 5.

e B and Ge as impurities are doped in the base region 4 to correct lattice distortion of the base region, thereby decreasing the band gap.

A manufacturing process of the semiconductor device shown in FIG. 55 will be described below.

1 As, Sb, P, or the like was doped in a p- or n-type substrate 1 by ion implantation or impurity diffusion to form an n⁺ -type buried region 2 (impurity concentration: 1×10¹⁶ to 10¹⁹ cm⁻³).

2 An n⁻ -type region 3 (impurity concentration: 1×10¹⁴ to 10¹⁷ cm⁻³) was formed by epitaxial techniques.

3 An n⁺ -type region 7 (impurity concentration: 1×10¹⁷ to 10²⁰ cm⁻³) was formed to decrease a collector resistance.

4 A channel stopper 6 was formed by ion implantation or thermal diffusion.

5 An element isolation region 101 was formed by selective oxidation or CVD.

6 A base region 4 was formed, Ge and B (BF₂, Gu, or the like may be used in place of B) were ion-implanted (3×10¹⁴ /cm² at 40 keV for, e.g., B), and annealing was performed at 900° C. for 20 minutes.

7 An emitter contact hole was formed in an oxide film 102, and an emitter region 5 was formed without being through a thin oxide film. In this embodiment, surface cleaning was performed by epitaxial techniques in an H₂ atmosphere at 900° C. and 10 Torr. Then, epitaxial growth was performed in an atmosphere of SiH₂ Cl₂ +H₂ at 900° C. and 50 Torr, thereby forming the emitter layer 5. In this case, the emitter opening portion consisted of single-crystal silicon. SiO₂ was converted into polysilicon.

8 An n-type impurity such as As or Ph was ion-implanted. in silicon (e.g., an impurity dose of 2.5×10¹⁴ cm⁻² at an acceleration voltage of 60 kV for Ph), and annealing was performed at 1,000° C. for 30 minutes. An n-type impurity was ion-implanted in the surface again, and rapid annealing was performed (1,000° C. for 30 minutes) so as not to disturb the impurity distribution of the emitter and base.

9 Additional annealing was performed, an insulating film 103 was deposited and then annealed, and contact holes are formed.

10 Al--Si was sputtered to form electrodes 200, and the Al--Si layer was patterned.

11 A passivation film 104 was formed after the Al--Si electrodes were alloyed.

The BPT shown in FIG. 55 was manufactured by the above steps. The most important steps in the present invention are step 6 of forming the base, step 9 of forming the single-crystal emitter, and step 8 of forming an impurity distribution in the emitter. These steps will be described in more detail below.

Step 6 of forming the base has effects wherein the shallow base is formed by forming a pre-amorphous base region upon ion implantation of Ge, and crystallization can be achieved upon doping of Ge. In order to obtain a high-speed BPT, the base must have a high impurity concentration and must be shallow.

In step 7 of forming the emitter, when a thin oxide film is formed on underlying silicon, good single-crystal silicon cannot be deposited. In this process, the natural oxide film is removed by a reduction action in a low-pressure H₂ atmosphere. However, surface cleaning may be performed by sputtering or the like using element ions in an HF gas or in a vacuum. In addition, film formation may be plasma CVD, CVD by sputtering, or the like. It is very important to suppress impurity diffusion from the base to the emitter during film formation. If the process is performed at 900° C. or less, this diffusion can be sufficiently suppressed.

In step 8 of forming the impurity distribution in the emitter, as a method of forming a region (LO) having a low impurity concentration and an surface area (HI) having a high impurity concentration from the emitter-base junction, an impurity doping method, annealing, or the like may be utilized to form the distribution.

Deposition of emitter Si, the impurity distribution, impurity activation, and the like can be performed by low-temperature annealing (less than about 900° C.) or a rapid annealing (e.g., lamp annealing and laser annealing).

The characteristics of the BPTs manufactured as described above were evaluated, and the current gains and frequency characteristics were improved.

(Embodiment 20)

FIG. 56 shows another embodiment of the present invention.

This embodiment is different from Embodiment 19 except that an emitter is embedded in an insulating film 102.

The characteristics of the BPT of this type were evaluated, and the same characteristics as in the BPT of Embodiment 19 could be exhibited.

(Embodiment 21)

A photoelectric transducer apparatus using the BPT of Embodiment 19 will be described as the embodiment 21 of the present invention.

FIG. 57 is a schematic circuit diagram of the photoelectric transducer apparatus of this embodiment.

This embodiment exemplifies a line sensor in which sensors S (S₁, S₂, . . . ) are aligned linearly.

Each sensor S consists of a bipolar transistor and a reset transistor Q_(res) connected to the base of the bipolar transistor. Carriers excited by light incident on the base of the bipolar transistor are accumulated and are read to the emitter. The reset transistor Q_(res) is turned on to reset the bipolar transistor to a predetermined potential.

A predetermined positive voltage is kept applied to the collector electrode of each sensor S, and emitter electrodes of the respective sensors S are connected to vertical lines L (L₁, L₂, . . . ).

A predetermined voltage V_(eg) is applied to each vertical line L through a transistor Q_(vrs) ' and a pulse .o slashed._(vrs) for controlling the ON/OFF operation is input to the gate electrode of the transistor Q_(vrs).

Each vertical line L is connected to an accumulation capacitor C_(t), and a signal is output from a transistor BPT₂ through the transistor Q_(t).

(Embodiment 22)

FIG. 41 is a circuit diagram in which the BPT shown in the embodiment 19 is used in a solid-state image pickup apparatus. The BPT shown in Embodiment 19 is used in a portion represented by Tr.

In this embodiment, the BPT is used as a photoelectric transducer element.

For example, when an area sensor shown in FIG. 11 is used as a color camera, optical information of the same photoelectric transducer element is read a plurality of times. In this case, since the optical information is read from the same element by a plurality of times, a ratio of the first read output to the second read output is very important. When this ratio is reduced, correction is required.

If the ratio of the first read output to the second read output is defined as a nondestructive degree, the nondestructive degree is represented by the following equation:

    Nondestructive Degree=(C.sub.tot ×h.sub.FE )/(C.sub.tot ×h.sub.FE +C.sub.V)

where C_(tot) is a total capacitance connected to the bases of the photoelectric transducer elements represented by Tr in FIG. 14. The total capacitance C_(tot) is determined by a base-emitter capacitance C, a base-collector capacitance C_(bc) and C_(ox). C_(V) is a stray capacitance of read lines represented by VL₁. . . VL_(n). Note that C_(ox) is not present depending on circuit systems.

The nondestructive degree can be easily improved by increasing the current gain h_(FE) . That is, when h_(FE) is increased, the nondestructive degree can be increased.

As has been described, according to the present Embodiments 19-22, the current gain and. the frequency characteristics of the semiconductor device can be greatly improved.

According to the present invention, therefore, there is provided a high-performance semiconductor device and a photoelectric transducer apparatus using the same. 

I claim:
 1. A semiconductor device comprising a first emitter region of a first conductivity type, a second emitter region formed of a polycrystalline semiconductor of the first conductivity type adjacent to said first emitter region, a base region of a second conductivity type adjacent to said first emitter region at a first side of said base region, and a collector region of the first conductivity type adjacent to a second side of said base region,wherein said first emitter region, and said base region include at least silicon and germanium atoms, said second emitter region includes at least silicon atoms, and said collector region includes a first sub-region including silicon and germanium atoms provided at a side adjacent to said base region, and second sub-region including silicon atoms but not including germanium atoms and adjacent to said first sub-region, and interface between said first and second sub-regions is within said collector region.
 2. A semiconductor device according to claim 1, wherein an impurity concentration of said first emitter region is 10¹⁷ to 10¹⁹ cm⁻³.
 3. A semiconductor device according to claim 1, wherein an impurity concentration of said second emitter region is 10¹⁷ to 10²¹ cm⁻³.
 4. A semiconductor device according to claim 1, wherein said base region comprises semiconductor Sil--xGex (0<x<1).
 5. A semiconductor device according to claim 1, wherein said base region includes boron atom, the boron concentration is identical with or less than the germanium concentration.
 6. A semiconductor device comprising a first emitter region of the first conductivity type, a second emitter region formed of a polycrystalline semiconductor adjacent to said first emitter region, a base region of second conductivity type adjacent to a first side of said first emitter region of the first conductivity type, a collector region of the first conductivity type adjacent to a second side of said base region, first and second regions with a predetermined energy band gap width, and a third region with an energy band gap width narrower than the energy band gap widths of said first and second regions and provided between said first and second regions,wherein said base region is formed within said third region, an interface between said first and third regions is formed within said emitter region, and an interface between said second and third regions is formed within said collector region.
 7. A semiconductor device according to claim 6, wherein an impurity concentration of said first emitter region is 10¹⁷ to 10¹⁹ cm⁻³.
 8. A semiconductor device according to claim 6, wherein an impurity concentration of said second emitter region is 10¹⁹ to 10⁻²¹ cm⁻³.
 9. A semiconductor device according to claim 6, wherein said base region comprises a semiconductor Sil--xGex (0<x<1).
 10. A semiconductor device according to claim 6, wherein said base region includes boron atom, and an impurity concentration of the boron is identical with or less than the germanium concentration.
 11. A semiconductor device comprising a first emitter region of a first conductivity type formed of a polycrystallene semiconductor a second emitter region of the first conductivity type adjacent to said first emitter region, a base region of a second conductivity type adjacent to said second emitter region at a first side of the base region, and a collector region of the first conductivity type adjacent to a second side of said base region, whereinfirst and second regions of a predetermined energy band gap widths, and a third region of an energy band gap width narrower than the energy band gap width of said first and second regions are provided, a first interface is formed between said first and third regions, a second interface is formed between said second and third regions, said first interface resides in said emitter region, and said second interface resides in said collector region.
 12. A semiconductor device according to claim 11, whereinthe energy band gap width is gradually changed at said first and/or second interface.
 13. A semiconductor device comprising a first emitter region of a first conductivity, a second emitter region of a first conductivity region formed of a polycrystallene semiconductor adjacent to said first emitter region, a base region of a second conductivity region adjacent to said second emitter region at a first side of said base region, and a collector region of the first conductivity region adjacent to a second side of said base region, whereinsaid first emitter region and said base region include silicon atom and germanium atom at least, said second emitter region includes silicon atom at least, and a heterojunction is formed within said emitter region.
 14. A semiconductor device according to claim 13, whereinsaid first emitter region includes an impurity of a concentration 10¹⁷ -10¹⁹ cm⁻³.
 15. A semiconductor device according to claim 13, whereinsaid second emitter region includes an impurity of a concentration 10¹⁹ -10²¹ cm⁻³.
 16. A semiconductor device according to claim 13, whereinsaid base region is formed from a semiconductor material Si_(1-X) Ge_(x) (0<x<1).
 17. A semiconductor device according to claim 13, whereinsaid base region includes boron atom of a concentration not higher than the germanium concentration.
 18. A semiconductor device comprising an emitter region of a first conductivity type, a base region of a second conductivity type adjacent to said emitter region at a first side of said base, and a collector region of the first conductivity adjacent to a second side of said base region, whereinfirst and second regions with predetermined energy band gap widths are provided, a third region with an energy band gap width narrower than the energy band gap widths of said first and second regions is provided between said first and second regions, said base region is formed within said third region, and an interface between said first and third regions is formed within said emitter region.
 19. A semiconductor device according to claim whereinsaid emitter region includes first emitter region including an impurity of a concentration 10¹⁷ -10¹⁹ cm⁻³.
 20. A semiconductor device according to claim 18, whereinsaid emitter region includes a second emitter region including an impurity at a concentration 10¹⁹ -10²¹ cm⁻³.
 21. A semiconductor device according to claim 18, whereinsaid base region is formed from a semiconductor material Si_(1-X) Ge_(x) (0<x<1).
 22. A semiconductor device according to claim 18, whereinsaid base region includes boron atom at a concentration not higher than the germanium concentration. 